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Posts: 1

# How does sas proc power calculate the sample size?

In SAS proc power, there is an example for Determining Required Sample Size for a Two-Sample t Test.

Here is the original code from SAS

proc power;

twosamplemeans

groupmeans   = (13 14) (13 14.5) (13 15)

stddev       = 1.2 1.7

groupweights = 1 | 1 2 3

power        = 0.9

ntotal       = .;

run;

I calculate the sample size by using formula, the result is always smaller the one obtained from SAS output.

For example, the first sample size I got is 60.466176 but SAS got 62.507429. Basically,  the fractional n total from SAS is always bigger then the one I calculated based on the common formula which is

n=4*stddev^2*(1.96+1.28)^2/(14-13). I want to know how SAS compute the sample size and base on what formula. I hope someone can help me to find it out. Thanks.

Super User
Posts: 23,754

## Re: How does sas proc power calculate the sample size?

The documentation has a good description:

SAS/STAT(R) 9.2 User's Guide, Second Edition

Proc Power>Details>Computational Methods and Formulas>Two Sample Means

The results from proc power are the total sample size for both groups that match the results from a web calculator as well:

Power/Sample Size Calculator

SAS : 64

UBC Calc: 62

All seem within the same ballpark, though not exactly the same.