You are considering testing a new drug that is supposed to facilitate learning in mentally retarded children.Based on preliminary research, you have some idea about the size of the drug's effect.Because of your work schedule you can either do the test with 15 subjects or with 20.You would like to only run 15 subjects, but if running 20 will make power at least 20% higher, you will run 20 subjects.

Calculate power for the following conditions:

   a. N = 15, a = .05_{1 tail}, P_real = 0.70. 

 b. N = 20, a = .05_{1 tail}, P_real = 0.70.  

c. Based on your calculations, how many subjects will you test?