Sample size for McNemar's test
Effect size is the difference in proportions between two observations of the outcome
The values to seek out in the literature are 1) the odds ratio or magnitude of the intervention on changing from one level of the categorical variable to the other and 2) the proportion of the population that will change as a result of the intervention. The articles should be theoretically or conceptually similar to the study of interest. Use the reported values in a sample size calculation for McNemar's test. This is called using an evidence-based measure of effect size.
For example, let's say researchers find high quality evidence that people are twice as likely to change group membership as a result of an intervention and that 75% of participants are expected to change. The odds ratio is 2.0 and the proportion of discordant group change (people that will change from one level of the categorical variable to the other level) is 75%.
The steps for calculating sample size for a McNemar's test in G*Power
2. Under the Test family drop-down menu, select Exact.
3. Under the Statistical test drop-down menu, select Proportions: Inequality, two dependent groups (McNemar).
4. Under the Type of power analysis drop-down menu, select A priori: Compute required sample size - given alpha, power, and effect size.
5. If there is a directional hypothesis, under the Tail(s) drop-down menu, select One.
6. If there is a non-directional hypothesis, under the Tail(s) drop-down menu, select Two.
7. In the Odds ratio box, enter the magnitude of the expected change in the form of an odds ratio. Example: "2.0"
8. Leave the alpha value at 0.05, unless researchers want to change the alpha value according to the current empirical or clinical context.
9. Enter ".80" into the Power (1-beta err prob) box, unless researchers want to change the power according to the current empirical or clinical context.
10. In the Prop discordant pairs box, enter the proportion of the population that will change as a result of the intervention. Example: ".75"
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