The finite population standard deviation of a variable provides a measure of the amount of variation in the corresponding attribute of the study population’s members, thus helping to describe the distribution of a study variable. Whether your survey is measuring crop yields, adult alcohol consumption, or the body mass index (BMI) of school children, a small population standard deviation is indicative of uniformity in the population, while a large standard deviation is indicative of a more diverse population.
Suppose you have data that were sampled according to some complex survey design. The SURVEYMEANS procedure enables you to estimate sample totals, means, and ratios, as well as the design-based variances of the estimated quantities, but it does not directly compute the standard deviation of a variable. However, because a standard deviation can be expressed mathematically as a function of a total, you can easily estimate the finite population standard deviation of a variable by using PROC SURVEYMEANS plus a little SAS programming.
Whenever you estimate a population parameter such as a mean or a standard deviation, you should also report the precision of the estimate. The most commonly reported measure of precision is the variance (or its square root, the standard error). The survey analysis procedures in SAS/STAT software currently provide three different variance estimation methods for complex survey designs: the Taylor series linearization method, the delete-one jackknife method, and the balanced repeated replication (BRR) method. This example demonstrates how to use all three methods to estimate the variance .
Suppose you want to estimate the standard deviation of a variable from a finite population by using data that were collected using some complex survey design. The finite population standard deviation of is
(1) |
where is the total number of elements in the population, is the th observation of the variable , and is the population mean of . A sample-based statistic of is
(2) |
where is an estimator of the population total , is an estimator of the population mean, is the number of elements in the sample, and is the probability that element is observed in the sample.
To estimate , you first estimate both and with PROC SURVEYMEANS. Next, you generate a variable (call it ) such that each observation is equal to
(3) |
Now you use PROC SURVEYMEANS to estimate the total of . The square root of the estimated weighted total of is equal to . Estimating , the variance of , requires some additional SAS programming.
To estimate by using the Taylor series linearization method, construct a variable , such that
(4) |
where is computed as in equation (2). Use PROC SURVEYMEANS to estimate the total (and the variance of the total) of . The total that is computed by PROC SURVEYMEANS is of no interest, but the variance of the total is equal to , the variance of the estimate (Särndal, Swensson, and Wretman 1992 , chap. 5.5).
The following steps summarize how you estimate , the finite population standard deviation of a variable , and , the variance of the finite population standard deviation estimator (using the Taylor series linearization method):
Use PROC SURVEYMEANS to estimate the sample mean of the variable , and save the estimated mean. PROC SURVEYMEANS also computes the sum of the sampling weights, which is the value of in the analysis. Save that value also; it is used in the construction of .
Using the sample mean from step 1, construct the variable as in equation (3).
Use PROC SURVEYMEANS to estimate the weighted total of the variable . Save the estimated total, which is the estimate of the population variance (). Take the square root of the weighted total. Save the result, which is the estimate of the finite population standard deviation.
Construct the variable as in equation (4).
Use PROC SURVEYMEANS to estimate the weighted total (and the variance of the total) of the variable . The estimated variance of this total obtained from PROC SURVEYMEANS is an estimator of the variance of .
This example uses the IceCreamStudy data set from the example "Stratified Cluster Sample Design" in the chapter "The SURVEYMEANS Procedure" of the SAS/STAT User's Guide.
The study population is a junior high school with a total of 4,000 students in grades 7, 8, and 9. In the original example, researchers want to know how much these students spend weekly for ice cream, on the average, and what percentage of students spend at least $10 weekly for ice cream. This example measures the variability of the students’ expenditures by estimating , the variance of the variable that contains the students’ expenditures.
Suppose that every student belongs to a study group and that study groups are formed within each grade level. Each study group contains between two and four students. Table 1 shows the total number of study groups and the total number of students for each grade.
Grade |
Number of Study Groups |
Number of Students |
---|---|---|
7 |
608 |
1,824 |
8 |
252 |
1,025 |
9 |
403 |
1,151 |
It is quicker and more convenient to collect data from students in the same study group than to collect data from students individually. Therefore, this study uses a stratified clustered sample design. The primary sampling units are study groups. The list of all study groups in the school is stratified by grade level. From each grade level, a sample of study groups is randomly selected, and all students in each selected study group are interviewed. The sample consists of eight study groups from the 7th grade, three groups from the 8th grade, and five groups from the 9th grade.
The SAS data set IceCreamStudy saves the responses of the selected students:
data IceCreamStudy; input Grade StudyGroup Spending Weight @@; datalines; 7 34 7 76.0 7 34 7 76.0 7 412 4 76.0 9 27 14 80.6 7 34 2 76.0 9 230 15 80.6 9 27 15 80.6 7 501 2 76.0 9 230 8 80.6 9 230 7 80.6 7 501 3 76.0 8 59 20 84.0 7 403 4 76.0 7 403 11 76.0 8 59 13 84.0 8 59 17 84.0 8 143 12 84.0 8 143 16 84.0 8 59 18 84.0 9 235 9 80.6 8 143 10 84.0 9 312 8 80.6 9 235 6 80.6 9 235 11 80.6 9 312 10 80.6 7 321 6 76.0 8 156 19 84.0 8 156 14 84.0 7 321 3 76.0 7 321 12 76.0 7 489 2 76.0 7 489 9 76.0 7 78 1 76.0 7 78 10 76.0 7 489 2 76.0 7 156 1 76.0 7 78 6 76.0 7 412 6 76.0 7 156 2 76.0 9 301 8 80.6 ;
Table 2 identifies the variables contained in the data set IceCreamStudy.
Variable |
Description |
---|---|
Grade |
Student’s grade (strata) |
StudyGroup |
Student’s study group (PSU) |
Spending |
Student’s expenditure per week for ice cream, in dollars |
Weight |
Sampling weights |
The SAS data set StudyGroup is created to provide PROC SURVEYMEANS with the sample design information shown in Table 1. The variable Grade identifies the strata, and the variable _TOTAL_ contains the total number of study groups in each stratum.
data StudyGroups; input Grade _total_; datalines; 7 608 8 252 9 403 ;
Use PROC SURVEYMEANS to obtain an estimate of the sample mean. Specify the MEAN and STACKING options in the PROC SURVEYMEANS statement. The STACKING option causes the procedure to create an output data set with a single observation. This table structure makes it easy in later steps to identify the saved estimates and to assign their values to macro variables. The WEIGHT statement specifies that the variable Weight contain the sampling weights. The STRATA statement specifies that the variable Grade identifies strata membership. The CLUSTER statement specifies that the variable StudyGroup identifies cluster (or PSU) membership. The ODS OUTPUT statement requests output data sets for the statistics and data summary tables, to be named Statistics and Summary, respectively. The sample mean is stored in the data set Statistics. The data set Summary contains the sum of the sampling weights, the number of strata, and the number of clusters. The sum of the sampling weights is needed to compute ; the number of strata and the number of clusters are used later to compute confidence limits for .
proc surveymeans data=IceCreamStudy mean stacking ; weight Weight; strata Grade; cluster StudyGroup; var Spending; ods output Statistics = Statistics Summary = Summary; run;
The following DATA step saves the sample mean of the variable Spending in a macro variable named Spending_Mean:
data _null_; set Statistics; call symput("Spending_Mean",Spending_Mean); run;
The next DATA step saves the sum of the sampling weights in a macro variable named N, the number of strata in a macro variable named H, and the number of clusters in a macro variable named C:
data Summary; set Summary; if Label1="Sum of Weights" then call symput("N",cValue1); if Label1="Number of Strata" then call symput("H",cValue1); if Label1="Number of Clusters" then call symput("C",cValue1); run;
Construct the variable in a DATA step by using the macro variables Spending_Mean and N:
data Working; set IceCreamStudy; z=(1/(&N-1))*(Spending-&Spending_Mean)**2; run;
Use PROC SURVEYMEANS to estimate the weighted total of the variable . Specify the SUM and STACKING options in the PROC SURVEYMEANS statement. The ODS OUTPUT statement saves the statistics table to a data set named Result.
proc surveymeans data = Working sum stacking; weight Weight; var z; ods output Statistics = Result; run;
The following DATA step retrieves the estimated total of and stores it in a macro variable named Variance. The total of is equal to . Take the square root of the estimated total and store it in a macro variable named StdDev. The square root of the estimated total is the finite population standard deviation .
data Result; set Result; StdDev=sqrt(z_Sum); call symput("Variance",z_Sum); call symput("StdDev",StdDev); run;
Construct the variable by using the macro variables Spending_Mean, N, Variance, and StdDev.
data Taylor; set IceCreamStudy; u=((Spending-&Spending_Mean)**2 - &Variance)/(2*&StdDev*(&N-1)); run;
Use PROC SURVEYMEANS to estimate the total of the variable . Specify the SUM, VARSUM, TOTAL=, and STACKING options in the PROC SURVEYMEANS statement. The VARSUM option computes the variance of the total. In this step, the computation of interest is the variance of the estimated total rather than the total itself. Therefore, the sampling design must be appropriately represented in the SURVEYMEANS procedure. The TOTAL= option enables the procedure to apply a finite population correction in the variance computation. The STRATA statement specifies that the strata be identified by the variable Grade, and the CLUSTER statement specifies that cluster membership be identified by the variable StudyGroup. The ODS OUTPUT statement saves the statistics table in a data set named Result.
proc surveymeans data = Taylor sum varsum stacking total=StudyGroups; strata Grade; cluster StudyGroup; weight Weight; var u; ods output Statistics = Result; run;
The following DATA step creates the variable Estimate in the data set Result and assigns it the value of that is stored in the macro variable StdDev. The confidence limits are computed, and the data set Result is prepared for printing.
%let df=%eval(&C - &H); data Result; set Result(rename=(u_VarSum=Variance u_StdDev=StdErr)); Estimate=&StdDev; LowerCL= Estimate + StdErr*TINV(.025,&df); UpperCL= Estimate + StdErr*TINV(.975,&df); label Estimate=Population Standard Deviation Estimate Variance=Variance of Estimate StdErr=Standard Error of Estimate LowerCL=Lower Confidence Limit UpperCL=Upper Confidence Limit; Variable='Spending'; run;
Use PROC PRINT to print the contents of the data set Result:
title 'Parameter Estimates'; proc print data=Result label noobs; var Variable Estimate Variance StdErr LowerCL UpperCL; run; title ;
Output 1 displays the results. The estimate of the population standard deviation of the variable Spending is 5.33. The variance of the estimate is 0.245. The standard error of the estimate is 0.49, and the estimated lower and upper 95% confidence limits are 4.27 and 6.40, respectively.
Parameter Estimates |
Variable | Population Standard Deviation Estimate |
Variance of Estimate | Standard Error of Estimate |
Lower Confidence Limit |
Upper Confidence Limit |
---|---|---|---|---|---|
Spending | 5.33483 | 0.244809 | 0.494782 | 4.26592 | 6.40374 |
The delete-one jackknife resampling method of variance estimation deletes one primary sampling unit (PSU) at a time from the full sample to create replicates, where is the total number of PSUs. In each replicate, the sample weights of the remaining PSUs are modified by the jackknife coefficient . The modified weights are called replicate weights.
If is the estimate of obtained using only the data and the replicate weights from the th replicate, the jackknife variance estimate is
(5) |
with degrees of freedom, where is the jackknife coefficient for the th replicate, is the number of replicates, and is the number of strata (or when there is no stratification). See the section Jackknife Method in the chapter "The SURVEYMEANS Procedure" of the SAS/STAT User's Guide for more details.
Recall that when you construct , you use estimates of and that are computed by using the full sample. However, the jackknife variance estimator requires that the be computed from the th replicate. Thus, the jackknife estimate of the variance of the total of is not equal to the jackknife estimate of the variance of .
The following steps summarize how you estimate , the finite population standard deviation of a variable , and , the variance of the finite population standard deviation estimator (using the delete-one jackknife method):
Use PROC SURVEYMEANS to estimate the sample mean and the sum of the weights for the full sample. Save both estimates as they are used in the construction of .
Construct as in equation (3), using the full-sample estimates of and obtained in step 1.
Use PROC SURVEYMEANS to estimate the weighted total of the variable . Take the square root of the total, and save the result, which is the full-sample estimate of the population standard deviation (). When you estimate the total, specify the VARMETHOD=JACKKNIFE option and the OUTWEIGHTS= and OUTJKCOEFS= method-options in the PROC SURVEYMEANS statement. Both the OUTWEIGHTS= and OUTJKCOEFS= data sets are used in later steps.
For each replicate, use PROC SURVEYMEANS to compute the sample mean and the sum of the weights by using only the data and replicate weights for the th replicate. Save the estimates for later use.
For each replicate, using the estimates for and that were obtained in step 4, construct the variable such that
(6) |
Use PROC SURVEYMEANS to estimate the weighted total of by replicate. Take the square root of each estimated total, and save the results for later use. The square root of the estimated weighted total of is equal to for the th replicate.
Construct a variable (call it ) by using the estimates from step 6, the jackknife coefficients, and the full-sample estimate from step 3 such that
Use PROC SURVEYMEANS to estimate the unweighted total of the variable from step 7. The estimated unweighted total of is , the delete-one jackknife estimate of the variance of .
This example uses the same IceCreamStudy data set that was described in the section Ice Cream Study Data Set and reproduces the steps described in the section Using the Delete-One Jackknife Method to Estimate . Steps 1 and 2 are identical to the first two steps in the previous example but are repeated here for completeness.
Use PROC SURVEYMEANS to obtain an estimate of the sample mean. Specify the MEAN and STACKING options in the PROC SURVEYMEANS statement. The WEIGHT statement specifies that the variable Weight contain the sampling weights. The STRATA statement specifies that the variable Grade identifies strata membership. The CLUSTER statement specifies that the variable StudyGroup identifies cluster (or PSU) membership. The ODS OUTPUT statement creates output data sets for the statistics and data summary tables, to be named Statistics and Summary, respectively. The sample mean is stored in the data set Statistics. The data set Summary contains the sum of the sampling weights and the number of strata.
proc surveymeans data=IceCreamStudy mean stacking ; weight Weight; strata Grade; cluster StudyGroup; var Spending; ods output Statistics = Statistics Summary = Summary; run;
The following DATA step saves the sample mean of the variable Spending in a macro variable named Spending_Mean:
data _null_; set Statistics; call symput("Spending_Mean",Spending_Mean); run;
The next DATA step saves the sum of the sampling weights in a macro variable named N and the number of strata in a macro variable named H:
data Summary; set Summary; if Label1="Sum of Weights" then call symput("N",cValue1); if Label1="Number of Strata" then call symput("H",cValue1); run;
Construct the variable in a DATA step using the macro variables Spending_Mean and N:
data Working; set IceCreamStudy; Z=(1/(&N-1))*(Spending-&Spending_Mean)**2; run;
Use PROC SURVEYMEANS to estimate the weighted total of the variable . Specify the SUM and STACKING options in the PROC SURVEYMEANS statement. Also specify the VARMETHOD=JACKKNIFE option with the OUTJKCOEFS= and OUTWEIGHTS= method-options. The OUTJKCOEFS= method-option saves the jackknife coefficients in a SAS data set named Jkcoefs. The OUTWEIGHTS= method-option saves the replicate weights in a SAS data set named Jkweights.
In this step you must fully specify the sampling design so that the jackknife coefficients and replicate weights are computed correctly. The STRATA statement specifies that the strata be identified by the variable Grade. The CLUSTER statement specifies that the PSUs be identified by the variable StudyGroup. The WEIGHT statement specifies that the full-sample sampling weights be contained in the variable Weight. The ODS OUTPUT statement saves the statistics table to a data set named Result and the variance estimation table to a data set named VarianceEstimation.
proc surveymeans data=Working sum stacking varmethod=JACKKNIFE(outjkcoefs=Jkcoefs outweights=Jkweights); strata Grade /list; cluster StudyGroup; weight Weight; var z; ods output Statistics = Result VarianceEstimation=VarianceEstimation; run;
You can see from the "Variance Estimation" table in Output 2 that there are 16 replicates.
Data Summary | |
---|---|
Number of Strata | 3 |
Number of Clusters | 16 |
Number of Observations | 40 |
Sum of Weights | 3162.6 |
Variance Estimation | |
---|---|
Method | Jackknife |
Number of Replicates | 16 |
The next DATA step retrieves the number of replicates and stores the value in a macro variable named R:
data _null_; set VarianceEstimation; where label1="Number of Replicates"; call symput("R",cvalue1); run; %let R=%eval(&R);
The data set Jkcoefs has 16 observations, one for each replicate. The th observation contains the jackknife coefficient for the th replicate. The data set Jkweights contains the original variables from the IceCreamStudy data set and 16 new variables named RepWgt_1 through RepWgt_16; there are observations.
The following DATA step retrieves the estimated total of the variable , takes the square root of the estimated total, and stores it in a macro variable named StdDev. The square root of the weighted total of the variable is .
data _null_; set Result; StdDev=sqrt(Z_Sum); call symput("StdDev",StdDev); run;
Before computing and , use the following DATA step to convert the data set Jkweights from wide form to long form; doing so enables you to use BY-group processing with PROC SURVEYMEANS.
data Long(drop= RepWt_1 - RepWt_&R Z); set Jkweights; array num (*) RepWt_1 - RepWt_&R; do replicate=1 to dim(num); Jkweight=num(replicate); output; end; run;
The data set Long has observations. There are 16 copies of the original variables from the IceCreamStudy data set stacked on top of each other, and each copy is identified by the variable Replicate. Instead of the 16 replicate weight variables, RepWgt_1 through RepWgt_16, there is now one variable, Jkweight, which is constructed by stacking the variables RepWgt_1 through RepWgt_16 on top of each other. Thus, the first 40 observations contain a copy of the original variables, the contents of RepWgt_1, and the variable Replicate has a value of 1. The second 40 observations contain a copy of the original variables, the contents of RepWgt_2, and the variable Replicate has a value of 2. The remaining observations are constructed and identified similarly.
Next, sort the data set Long by Replicate:
proc sort data=Long out=Long; by Replicate; run;
Use PROC SURVEYMEANS to estimate the mean of Spending by Replicate. Doing so produces the estimates of and for each replicate. The WEIGHT statement specifies that the sampling weights be contained in the variable Jkweight. The ODS OUTPUT statement saves the sample means () in a SAS data set named JKMeans and the sums of the replicate weights () in a data set named JKN. By default, the means are stored in a variable named Mean and the sums of the replicate weights are stored in a variable named N.
proc surveymeans data=Long mean; weight Jkweight; var Spending; by Replicate; ods output Statistics = JKMeans(keep=Replicate Mean) Summary = JKN; run;
Before you can construct the variable for the replicate samples, you must merge the data sets JKMeans and JKN with Long, by Replicate:
proc sort data=JKMeans out=JKMeans; by Replicate; run;
data JKN(keep=N replicate ); set JKN(rename=(nvalue1=N)); where Label1="Sum of Weights"; run;
proc sort data=JKN out=JKN; by Replicate; run;
data Long; merge Long JKN JKMeans; by Replicate; run;
Now construct the variable using the merged data set.
data Long; set Long; z=(1/(N-1))*(Spending-Mean)**2; run;
Use PROC SURVEYMEANS to estimate the total of the variable by Replicate. The WEIGHT statement specifies that the sampling weights be contained in the variable Jkweight. You do not need to specify the STRATA and CLUSTER statements. The ODS OUTPUT statement saves the estimated totals in the variable JKEstimate in a SAS data set named Statistics. The estimated totals are the estimates for each replicate.
proc surveymeans data=Long sum stacking; weight Jkweight; var z; by Replicate; ods output Statistics=Statistics(rename=(Z_Sum=JKEstimate)); run;
Take the positive square roots of the estimated totals. The results are the estimates for each replicate.
data Statistics; set Statistics(drop=Z_StdDEV z); JKEstimate=sqrt(JKEstimate); run;
Before you can construct the variable , you must sort and merge, by Replicate, the data sets Statistics and Jkcoefs:
proc sort data=Statistics out=Statistics; by Replicate; run;
proc sort data=Jkcoefs out=Jkcoefs; by Replicate; run;
data Statistics; merge Statistics Jkcoefs; by Replicate; run;
The data set Statistics now contains the jackknife coefficients in the variable JKcoefficients and the estimates in the variable JKEstimate. Construct the variable by using these variables and the full-sample estimate that is saved in the macro variable StdDev.
data Statistics; set Statistics; u=JKcoefficient*(JKEstimate-&StdDev)**2; run;
Use PROC SURVEYMEANS to compute the unweighted total of . Specify the SUM option in the PROC SURVEYMEANS statement. The ODS OUTPUT statement saves the total in a variable named Variance in a SAS data set named Result.
proc surveymeans data=Statistics sum; var u; ods output Statistics=Result(rename=(sum=Variance)); run;
The following DATA step computes the standard error of the estimate and the upper and lower 95% confidence limits. In this example, the confidence limits are computed using a distribution with degrees of freedom. The variable Estimate is generated and assigned the estimated value of that is stored in the macro variable StdDev. Labels are created for the existing variables, a new variable Variable is generated, and its value is specified to be the name of the variable that is being analyzed (Spending).
%let df=%eval(&R-&H); data Result; set Result; StdErr=sqrt(Variance); Estimate=&StdDev; UpperCL=Estimate + StdErr*TINV(.975,&df); LowerCL=Estimate + StdErr*TINV(.025,&df); label Estimate=Population Standard Deviation Estimate Variance=Variance of Estimate StdErr=Standard Error of Estimate LowerCL=Lower Confidence Limit UpperCL=Upper Confidence Limit; Variable='Spending'; run;
Use the PRINT procedure to print the contents of the data set Result:
title 'Parameter Estimates'; proc print data=Result label noobs; var Variable Estimate Variance StdErr LowerCL UpperCL; run; title ;
Output 3 displays the results. The estimate of the population standard deviation for the variable Spending is 5.33. The variance of the estimate is 0.27, and the standard error of the estimate is 0.52. The estimated lower and upper 95% confidence limits are 4.21 and 6.46, respectively.
Parameter Estimates |
Variable | Population Standard Deviation Estimate |
Variance of Estimate | Standard Error of Estimate |
Lower Confidence Limit |
Upper Confidence Limit |
---|---|---|---|---|---|
Spending | 5.33483 | 0.271465 | 0.52102 | 4.20923 | 6.46043 |
The BRR method requires that the full sample be drawn by using a stratified sample design with two PSUs per stratum. If is the total number of strata, the total number of replicates is the smallest multiple of four that is greater than . Each replicate is obtained by deleting one PSU per stratum according to the corresponding Hadamard matrix and adjusting the original weights for the remaining PSUs. The new weights are called replicate weights.
If is the estimate of obtained by using only the data and the replicate weights from the th replicate, the BRR variance estimate is
(7) |
with degrees of freedom. See the section Balanced Repeated Replication (BRR) Method in the chapter "The SURVEYMEANS Procedure" of the SAS/STAT User's Guide for more details.
Recall that when you construct , you use estimates of and that are computed by using the full sample. However, the BRR variance estimator requires that the be computed from the th replicate. Thus, the BRR estimate of the variance of the total of is not equal to the BRR estimate of the variance of .
The following steps summarize how you estimate , the finite population standard deviation of a variable , and , the variance of the finite population standard deviation estimator (using the BRR method):
Use PROC SURVEYMEANS to estimate the sample mean and the sum of the weights for the full sample. Save both estimates for later use: they are used in the construction of . Also save the number of strata for later use.
Construct as in equation (3) by using the full-sample estimates of and obtained in step 1.
Use PROC SURVEYMEANS to estimate the weighted total of the variable , take the square root of the estimated total, and save the result. The square root of the estimated total is the full-sample estimate of the population standard deviation (). When you estimate the total, specify the VARMETHOD=BRR option and the OUTWEIGHTS= method-option in the PROC SURVEYMEANS statement. The OUTWEIGHTS= SAS data set is used in later steps. Also save the number of replicates for later use.
For each replicate, use PROC SURVEYMEANS to estimate the sample mean and the sum of the weights by using only the data and replicate weights for the th replicate. Save the estimates for later use.
For each replicate, using the estimates for and that were obtained in step 4, construct the variable such that
(8) |
Use PROC SURVEYMEANS to estimate the weighted total of by replicate, take the positive square root of each estimated total, and save the results for later use. The square root of the estimated weighted total of is equal to for the th replicate.
Construct a variable (call it ) by using the estimates from step 6, the number of replicates , and the full-sample estimate from step 3 such that
Use PROC SURVEYMEANS to estimate the unweighted total of the variable from step 7. The estimated unweighted total of is , the BRR estimate of the variance of .
This example uses the MUNIsurvey data set from the section Variance Estimation Using Replication Methods in the chapter "The SURVEYMEANS Procedure" of the SAS/STAT User's Guide. The data are not shown here, but a SAS program that generates the data is included in the sample SAS code that you can download for this example.
In the original example, the San Francisco Municipal Railway (MUNI) conducted a survey to estimate the average waiting time for MUNI subway system’s passengers. This example estimates the standard deviation of the passengers’ waiting time.
The study uses a stratified cluster sample design. Each MUNI subway line is a stratum. The subway lines included in the study are 'J-Church,' 'K-Ingleside,' 'L-Taraval,' 'M-Ocean View,' 'N-Judah,' and the street car 'F-Market & Wharves.' The MUNI vehicles in service for these lines during a day are the primary sampling units. Within each stratum, two vehicles (PSUs) are randomly selected. Then the waiting times of passengers for a selected MUNI vehicle are collected.
The collected data are saved in the SAS data set MUNIsurvey. Table 3 identifies the variables contained in the data set.
Variable |
Description |
---|---|
Line |
The MUNI line that a passenger is riding (strata) |
Vehicle |
The vehicle that a passenger is boarding (PSU) |
Waittime |
The time (in minutes) that a passenger waited |
Weight |
Sampling weights |
Use PROC SURVEYMEANS to obtain estimates of the sample mean () and the sum of the sampling weights () for the full sample. Specify the MEAN and STACKING options in the PROC SURVEYMEANS statement. The WEIGHT statement specifies that the sampling weights be contained in the variable Weight. The STRATA statement specifies that the strata be identified by the variable Line. The CLUSTER statement specifies that the PSUs be identified by the variable Vehicle. The ODS OUTPUT statement produces output data sets for the statistics and data summary tables, to be named Statistics and Summary, respectively. The sample mean is stored in the data set Statistics. The sum of the sampling weights and the number of strata are stored in the data set Summary.
proc surveymeans data=MUNIsurvey mean stacking ; weight Weight; strata Line; cluster Vehicle; var Waittime; ods output Statistics = Statistics Summary = Summary; run;
The following DATA step saves the sample mean () of the variable Waittime in a macro variable named Waittime_Mean:
data _null_; set Statistics; call symput("Waittime_Mean",Waittime_Mean); run;
The next DATA step saves the sum of the sampling weights in a macro variable named N and the number of strata in a macro variable named H:
data Summary; set Summary; if Label1="Sum of Weights" then call symput("N",cValue1); if Label1="Number of Strata" then call symput("H",cValue1); run;
Construct the variable in a DATA step by using the macro variables Waittime_Mean and N:
data Working; set MUNIsurvey; Z=(1/(&N-1))*(Waittime-&Waittime_Mean)**2; run;
Use PROC SURVEYMEANS to estimate the total of the variable . Specify the SUM and STACKING options in the PROC SURVEYMEANS statement. Also specify the VARMETHOD=BRR OUTWEIGHTS= method-options. The OUTWEIGHTS= method-option saves the replicate weights in a SAS data set named BRRweights.
In this step you must fully specify the sampling design so that the replicate weights are computed correctly. The STRATA statement specifies that the strata be identified by the variable Line. The CLUSTER statement specifies that the PSUs be identified by the variable Vehicle. The WEIGHT statement specifies that the full-sample sampling weights be contained in the variable Weight. The ODS OUTPUT statement saves the statistics table to a data set named Estimate and the variance estimation table to a data set named VarianceEstimation.
proc surveymeans data=Working sum stacking varmethod=brr(outweights=BRRweights); strata Line; cluster Vehicle; weight Weight; var z; ods output Statistics = Estimate VarianceEstimation=VarianceEstimation; run;
Data Summary | |
---|---|
Number of Strata | 6 |
Number of Clusters | 12 |
Number of Observations | 1937 |
Sum of Weights | 143040 |
Variance Estimation | |
---|---|
Method | BRR |
Number of Replicates | 8 |
There are observations and replicates. The data set BRRweights contains the original variables from the Munisurvey data set and eight new variables named RepWgt_1 through RepWgt_8.
The following DATA step retrieves the estimated total of the variable , takes the square root of the total, and stores the result in a macro variable named StdDev. The square root of the total of the variable is equal to .
data _null_; set Estimate; StdDev=sqrt(Z_Sum); call symput("StdDev",StdDev); run;
The next DATA step retrieves the number of replicates and stores the value in a macro variable named R:
data _null_; set VarianceEstimation; where label1="Number of Replicates"; call symput("R",cvalue1); run; %let R=%eval(&R);
Before computing and , use the following DATA step to convert the data set BRRweights from wide form to long form; doing so enables you to use BY-group processing with PROC SURVEYMEANS.
data Long(drop= RepWt_1 - RepWt_&R Z); set BRRweights; array num (*) RepWt_1 - RepWt_&R; do replicate=1 to dim(num); BRRweight=num(replicate); output; end; run;
The data set Long has observations. There are eight copies of the original variables from the Munisurvey data set stacked on top of each other, and each copy is identified by the variable Replicate. Instead of the eight replicate weight variables, RepWgt_1 through RepWgt_8, there is now one variable, BRRweight, which is constructed by stacking the variables RepWgt_1 through RepWgt_8 on top of each other. Thus, the first 1,937 observations contain a copy of the original variables and the contents of RepWgt_1, and the variable Replicate has a value of 1. The second 1,937 observations contain a copy of the original variables and the contents of RepWgt_2, and the variable Replicate has a value of 2. The remaining observations are constructed and identified similarly.
Next, sort the data set Long by Replicate:
proc sort data=Long out=Long; by Replicate; run;
Use PROC SURVEYMEANS to estimate the mean of Waittime by Replicate. Doing so produces the estimates of and for each replicate. The WEIGHT statement specifies that the sampling weights be contained in the variable BRRweight. The ODS OUTPUT statement saves the sample means in a SAS data set named BRRMeans and the sum of the replicate weights in a data set named BRRN.
proc surveymeans data=Long mean; weight BRRweight; var Waittime; by Replicate; ods output Statistics = BRRMeans(keep=Replicate Mean) Summary = BRRN; run;
Before you can construct the variable , you must merge the data sets BRRMeans and BRRN with Long by Replicate:
proc sort data=BRRMeans out=BRRMeans; by Replicate; run;
data BRRN(keep=N replicate ); set BRRN(rename=(nvalue1=N)); where Label1="Sum of Weights"; run;
proc sort data=BRRN out=BRRN; by Replicate; run;
data Long; merge Long BRRN BRRMeans; by Replicate; run;
Now construct the variable using the merged data set:
data Long; set Long; z=(1/(N-1))*(Waittime-Mean)**2; run;
Use PROC SURVEYMEANS to estimate the total of the variable by Replicate. The WEIGHT statement specifies that the sampling weights be contained in the variable BRRweight. You do not need to specify the STRATA and CLUSTER statements. The ODS OUTPUT statement saves the estimated totals in the variable BRREstimate in a SAS data set named Statistics. The estimated totals are the estimates for each replicate.
proc surveymeans data=Long sum stacking; weight BRRweight; var z; by Replicate; ods output Statistics=Statistics(rename=(Z_Sum=BRREstimate)); run;
Take the square root of each estimated total. The results are the estimates for each replicate.
data Statistics; set Statistics(drop= Z_StdDEV z); BRREstimate=sqrt(BRREstimate); run;
data Statistics; set Statistics; u=(1/&R)*(BRREstimate-&StdDev)**2; run;
Use PROC SURVEYMEANS to compute the unweighted total of . Specify the SUM option in the PROC SURVEYMEANS statement. The ODS OUTPUT statement saves the total in a variable named Variance in a SAS data set named Result.
proc surveymeans data=Statistics sum; var u; ods output Statistics=Result(rename=(sum=Variance)); run;
The following DATA step computes the standard error of the estimate and the upper and lower 95% confidence limits. The confidence limits for this example are computed by using a distribution with H=6 degrees of freedom. The variable Estimate is generated and assigned the estimated value of , which is stored in the macro variable StdDev. The data set is also prepared for printing.
data Result; set Result; StdErr=sqrt(Variance); Estimate=&StdDev; UpperCL=Estimate + StdErr*TINV(.975,&H); LowerCL=Estimate + StdErr*TINV(.025,&H); Variable='Waittime'; label Estimate=Population Standard Deviation Estimate Variance=Variance of Estimate StdErr=Standard Error of Estimate LowerCL=Lower Confidence Limit UpperCL=Upper Confidence Limit; run;
Use the PRINT procedure to print the contents of the data set Result:
title 'Parameter Estimates'; proc print data=Result label noobs; var Variable Estimate Variance StdErr LowerCL UpperCL; run; title ;
Output 5 displays the results. The estimate of the population standard deviation for the variable Waittime is 4.24. The variance of the estimate is 0.03, and the standard error of the estimate is 0.17. The estimated lower and upper 95% confidence limits are 3.82 and 4.67, respectively.
Parameter Estimates |
Variable | Population Standard Deviation Estimate |
Variance of Estimate | Standard Error of Estimate |
Lower Confidence Limit |
Upper Confidence Limit |
---|---|---|---|---|---|
Waittime | 4.24495 | 0.029935 | 0.17302 | 3.82159 | 4.66831 |