Hello,
How would you estimate the required sample size for a 2x4 ANOVA using G-Power based on previous research. I am particularly struggling to understand how to define 'number of groups' and 'number of measurements'.
Thank you!
Hello Skomantas,
Using "F tests" family, "ANOVA fixed effects, special, main effects and interactions:"
The number of groups is 8 (a.k.a. cells = 2 x 4), though this has no real import as far as your scenario is concerned: it's the df for the target hypothesis test that really matters.
The numerator df for the main effect of factor "A" (the two-level factor) is 1;
The numerator df for the main effect of factor "B" (the four-level factor) is 3;
The numerator df for the interaction of "A" and "B" is 3.
Good luck with your work.
you can use statistical power calculator. One of the most commonly used is G*Power. You have to calculate adequate sample size for Two way ANOVA using this free software. As a general guideline, you can calculate power .80, alpha= 0.05 , effect size as medium ( f= .25). however, we have to know your number of groups to exact number of sample size. For example using above guideline, gender (male, female) and grade level ( 1 to 4). In this example you have eight groups.
Step by step in G* Power:
download g power using this address: http://gpower.hhu.de/
Install your computer.
Choose test family as F-test,
Choose statistical test as ANOVA: Fixed effects special, main effects and interactions
Choose type of power analysis as A priori: Compute required sample size, given alpha and power.
write input parameter below
power .80, alpha= 0.05 , effect size as medium ( f= .25), number of groups (8 )
numerator df as 10. (Note: Program also give you most commonly used statistic when you open)
click calculate
the program give you required sample size, power and critical F value.
That is all.
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