/*  (Programmed by Phil Chapman; revised 9-16-96; send comments or

     corrections to pchapman@lamar.colostate.edu)

    This program calculates power for one sample (two-sided) t-tests.

    The null hypothesis that mu is  equal to mu0

    is tested versus the alternative that mu is not equal to mu0.

    User input values:

    1. alpha:    the type I error probability.

    2. sigma:    the standard deviation of the responses.

    3. mu0       the hypothesized mean.

    4. mu1       the true alternative mean.

    5. n1,n2,n3  the sample size minimum, maximum and increment.  For

                 example 2,20,2 would calculate power for even sample sizes from

                 2 to 20.  n1 must be at least 2.



    To run the program change the values below to the desired values and run the

    program like any other SAS program.  Be sure that you do not type over

    the semi-colons.  The output of the program is a plot and a table, both

    reporting the same results.  The program often gives error messages when the

    power is very close to 1.0;

*/



data powert;

* BEGIN USER INPUTS;

   alpha=0.05;

   sigma=4.0;

   mu0=12;

   mu1=16;

   n1=4;         *n1 must be at least 2;

   n2=20;

   n3=2;

* END USER INPUTS;

   do n=n1 to n2 by n3;

      tcrit=tinv(1-alpha/2,n-1);

      lambda=abs(mu1-mu0)/(sigma/sqrt(n));

      power=probt(-tcrit,n-1,lambda)+(1-probt(tcrit,n-1,lambda));

      output;

      end;

proc print;

   var tcrit sigma mu0 mu1 n power;

proc plot;

   plot power*n;

run;