Let’s modify Exercise 15.12 to have sample means of 25 and 28, with a pooled standard deviation of 8 and sample sizes of 20 and 20.

a) What is the a priori power of this experiment?

b) Run the t test on the data.

c) What, if anything, does the answer to

(a) have to say about the answer to (b)?

Exercise 15.12

Assume that I have just conducted a study comparing cognitive development of low birth weight

(premature) and normal birth weight babies at one year of age. Using a score of my own devising, I found the sample means of the two groups to be 25 and 30, respectively, with a pooled standard deviation (sp) of 8. There were 20 subjects in each group. If we assume that the true means and standard deviations have been estimated exactly, what was the a priori probability (the probability before the experiment was conducted) that this study would find a significant difference?