Section 3: A Priori Power Analysis

In G*Power, now select:

Type of power analysis = A priori: Compute required sample size.

Input effect size d from Section 1.

Specify α err prob.

Specify Power (1 - β) = .80.

Set the Allocation ratio to 1 (that is, equal sample sizes).

Click Calculate.

Provide a screenshot of your G*Power output. Interpret the meaning of a .80 power value. Specifically, report the estimated n1, n2, and total N to achieve obtain a power of .80. How many total subjects ( N) would be needed to obtain a power of .80? Would you have expected a required N of this size? Why or why not?

Next, in G*Power, change the Cohen's d effect size value obtained in Section 1 and set it to .50 (conventional "medium" effect size). Click Calculate. How many total subjects ( N) are needed to obtain a power of .80? Compare and contrast these two estimated Ns.

In conclusion, reflect on the importance of conducting an a priori power analysis in psychological research plans.

Resources

Power Analysis Scoring Guide.

BP Study Dataset.

Computing Power and Sample Size.

Data Analysis and Application Template.

G*Power 3.