TWOSAMPLEMEANS
<options> ;
The TWOSAMPLEMEANS statement performs power and sample size analyses for pooled and unpooled t tests, equivalence tests, and confidence interval precision involving two independent samples.
Table 71.21 summarizes the options available in the TWOSAMPLEMEANS statement.
Table 71.21: TWOSAMPLEMEANS Statement Options
Option 
Description 

Define analysis 

Specifies an analysis of precision of the confidence interval 

Specifies the underlying distribution assumed for the test statistic 

Specifies the statistical analysis 

Specify analysis information 

Specifies the significance level 

Specifies the lower equivalence bound 

Specifies the null mean difference 

Specifies the null mean ratio 

Specifies the number of sides and the direction of the statistical test or confidence interval 

Specifies the upper equivalence bound 

Specify effects 

Specifies the desired confidence interval halfwidth 

Specifies the two group means 

Specifies the mean difference 

Specifies the geometric mean ratio, 

Specify variability 

Specifies the common coefficient of variation 

Specifies the standard deviation of each group 

Specifies the common standard deviation 

Specify sample size and allocation 

Specifies the two group sample sizes 

Specifies the sample size allocation weights for the two groups 

Enables fractional input and output for sample sizes 

Specifies the common sample size per group 

Specifies the sample size 

Specify power and related probabilities 

Specifies the desired power of the test 

Specifies the type of probability for the PROBWIDTH= option 

Specifies the desired probability of obtaining a confidence interval halfwidth less than or equal to the value specified 

Control ordering in output 

Controls the output order of parameters 
Table 71.22 summarizes the valid result parameters for different analyses in the TWOSAMPLEMEANS statement.
Table 71.22: Summary of Result Parameters in the TWOSAMPLEMEANS Statement
Analyses 
Solve For 
Syntax 

TEST=DIFF 
Power 

Sample size 

Group sample size 
GROUPNS= n1  . 

GROUPNS=.  n2 

GROUPNS= (n1 .) 

GROUPNS= (. n2) 

Group weight 
GROUPWEIGHTS= w1  . 

GROUPWEIGHTS=.  w2 

GROUPWEIGHTS= (w1 .) 

GROUPWEIGHTS= (. w2) 

Alpha 

Group mean 
GROUPMEANS= mean1  . 

GROUPMEANS=.  mean2 

GROUPMEANS= (mean1 .) 

GROUPMEANS= (. mean2) 

Mean difference 

Standard deviation 

TEST=DIFF_SATT 
Power 

Sample size 

TEST=RATIO 
Power 

Sample size 

TEST=EQUIV_DIFF 
Power 

Sample size 

TEST=EQUIV_RATIO 
Power 

Sample size 

CI=DIFF 
Prob(width) 

Sample size 

To define the analysis, choose one of the following parameterizations:
To specify the means, choose one of the following parameterizations:
individual group means (by using the GROUPMEANS= option)
mean difference (by using the MEANDIFF= option)
mean ratio (by using the MEANRATIO= option)
To specify standard deviations in the Satterthwaite t test (TEST=DIFF_SATT), choose one of the following parameterizations:
common standard deviation (by using the STDDEV= option)
individual group standard deviations (by using the GROUPSTDDEVS== option)
To specify the sample sizes and allocation, choose one of the following parameterizations:
sample size per group in a balanced design (by using the NPERGROUP= option)
total sample size and allocation weights (by using the NTOTAL= and GROUPWEIGHTS= options)
individual group sample sizes (by using the GROUPNS= option)
This section summarizes the syntax for the common analyses supported in the TWOSAMPLEMEANS statement.
You can use the NPERGROUP= option in a balanced design and express effects in terms of the mean difference, as in the following statements. Default values for the DIST=, SIDES=, NULLDIFF=, and ALPHA= options specify a twosided test for no difference with a normal distribution and a significance level of 0.05.
proc power; twosamplemeans test=diff meandiff = 7 stddev = 12 npergroup = 50 power = .; run;
You can also specify an unbalanced design by using the NTOTAL= and GROUPWEIGHTS= options and express effects in terms of individual group means:
proc power; twosamplemeans test=diff groupmeans = 8  15 stddev = 4 groupweights = (2 3) ntotal = . power = 0.9; run;
Another way to specify the sample sizes is with the GROUPNS= option:
proc power; twosamplemeans test=diff groupmeans = 8  15 stddev = 4 groupns = (25 40) power = .; run;
The following statements demonstrate a power computation for the twosample Satterthwaite t test allowing unequal variances. Default values for the DIST=, SIDES=, NULLDIFF=, and ALPHA= options specify a twosided test for no difference with a normal distribution and a significance level of 0.05.
proc power; twosamplemeans test=diff_satt meandiff = 3 groupstddevs = 5  8 groupweights = (1 2) ntotal = 60 power = .; run;
The following statements demonstrate a power computation for the pooled t test of a lognormal mean ratio. Default values for the DIST=, SIDES=, NULLRATIO=, and ALPHA= options specify a twosided test of mean ratio = 1 assuming a lognormal distribution and a significance level of 0.05.
proc power; twosamplemeans test=ratio meanratio = 7 cv = 0.8 groupns = 50  70 power = .; run;
The following statements demonstrate a sample size computation for the TOST equivalence test for a normal mean difference. A default value of GROUPWEIGHTS=(1 1) specifies a balanced design. Default values for the DIST= and ALPHA= options specify a significance level of 0.05 and an assumption of normally distributed data.
proc power; twosamplemeans test=equiv_diff lower = 2 upper = 5 meandiff = 4 stddev = 8 ntotal = . power = 0.9; run;
The following statements demonstrate a power computation for the TOST equivalence test for a lognormal mean ratio. Default values for the DIST= and ALPHA= options specify a significance level of 0.05 and an assumption of lognormally distributed data.
proc power; twosamplemeans test=equiv_ratio lower = 3 upper = 7 meanratio = 5 cv = 0.75 npergroup = 50 power = .; run;
By default CI=DIFF analyzes the conditional probability of obtaining the desired precision, given that the interval contains the true mean difference, as in the following statements. The defaults of SIDES=2 and ALPHA=0.05 specify a twosided interval with a confidence level of 0.95.
proc power; twosamplemeans ci = diff halfwidth = 4 stddev = 8 groupns = (30 35) probwidth = .; run;