Simple Random Sampling

The following PROC SURVEYSELECT statements select a probability sample of customers from the Customers data set by using simple random sampling:

title1 'Customer Satisfaction Survey';
title2 'Simple Random Sampling';
proc surveyselect data=Customers method=srs n=100

The PROC SURVEYSELECT statement invokes the procedure. The DATA= option names the SAS data set Customers as the input data set from which to select the sample. The METHOD=SRS option specifies simple random sampling as the sample selection method. In simple random sampling, each unit has an equal probability of selection, and sampling is without replacement. Without-replacement sampling means that a unit cannot be selected more than once. The N= option specifies a sample size of 100 customers. The OUT= option stores the sample in the SAS data set named SampleSRS.

Figure 99.2 displays the output from PROC SURVEYSELECT, which summarizes the sample selection. A sample of 100 customers is selected from the data set Customers by simple random sampling. With simple random sampling and no stratification in the sample design, the selection probability is the same for all units in the sample. In this sample, the selection probability for each customer equals 0.007423, which is the sample size (100) divided by the population size (13,471). The sampling weight equals 134.71 for each customer in the sample, where the weight is the inverse of the selection probability. If you specify the STATS option, PROC SURVEYSELECT includes the selection probabilities and sampling weights in the output data set. (This information is always included in the output data set for more complex designs.)

The random number seed is 39647. PROC SURVEYSELECT uses this number as the initial seed for random number generation. Because the SEED= option is not specified in the PROC SURVEYSELECT statement, the seed value is obtained by using the time of day from the computer’s clock. You can specify SEED=39647 to reproduce this sample.

Figure 99.2: Sample Selection Summary

Customer Satisfaction Survey
Simple Random Sampling


Selection Method Simple Random Sampling

Input Data Set CUSTOMERS
Random Number Seed 39647
Sample Size 100
Selection Probability 0.007423
Sampling Weight 134.71
Output Data Set SAMPLESRS

The sample of 100 customers is stored in the SAS data set SampleSRS. PROC SURVEYSELECT does not display this output data set. The following PROC PRINT statements display the first 20 observations of SampleSRS:

title1 'Customer Satisfaction Survey';
title2 'Sample of 100 Customers, Selected by SRS';
title3 '(First 20 Observations)';
proc print data=SampleSRS(obs=20);

Figure 99.3 displays the first 20 observations of the output data set SampleSRS, which contains the sample of customers. This data set includes all the variables from the DATA= input data set Customers. If you do not want to include all variables, you can use the ID statement to specify which variables to copy from the input data set to the output (sample) data set.

Figure 99.3: Customer Sample (First 20 Observations)

Customer Satisfaction Survey
Sample of 100 Customers, Selected by SRS
(First 20 Observations)

Obs CustomerID State Type Usage
1 017-27-4096 GA New 168
2 026-37-3895 AL New 59
3 038-54-9276 GA New 785
4 046-40-3131 FL New 60
5 070-37-6924 GA New 524
6 100-58-3342 FL New 302
7 107-61-9029 AL New 235
8 110-95-0432 FL New 12
9 112-81-9251 SC New 347
10 137-33-0478 GA New 551
11 143-83-4677 AL New 203
12 147-19-9164 GA New 172
13 159-51-0606 FL New 102
14 164-14-7799 GA Old 388
15 165-05-7323 SC New 606
16 174-69-3566 AL Old 111
17 177-69-6934 FL New 202
18 181-58-3508 AL Old 261
19 207-41-8446 AL Old 183
20 207-64-7308 GA New 193