in research papers, I keep coming across Orme's rule of thumb for calculating the sample size for a CBC. I would like to know what role does the number of questionnaire versions play in using the formula? Does it only apply when using one questionnaire version?
As an example, with 75 respondents, 10 tasks, 3 concepts, 3 levels, the formula yields 750 level appearances and is below the recommended of 1000 times. Increasing the tasks to 20 (and blocked into 2 questionnaire versions) would yield 1500. The reason I ask is because we have a realtively small sample (n=75) in a current project. However, taking blocking into account in the formula would thus easily yield the recommended number of appearances even with a small sample.
Or what am I seeing wrong?