Computes the sample size required to estimate an intraclass correlation with desired confidence interval precision. This type of intraclass correlation can be used to describe the reliability of a single measurement (e.g., a single rater or a single form of a test). This intraclass correlation assumes a two-factor (subject x measurement) model. Use the size.ci.cronbach function to determine the sample size required to estimate the reliability of a sum or average of r measurements (e.g., items) with desired confidence interval precision.
Details
Specifying an intraclass correlation planning value for a conservatively large sample size requirement is not straightforward with an intraclass correlation. For r = 2, the sample size requirement is largest for an intraclass correlation of zero. But for r = 3, 4, 5, 10, 20, and 40 an intraclass correlation planning value of about .26, .33, .38, .43, .44, and .45, respectively, maximizes the sample size requirement.
References
Bonett DG (2002). “Sample size requirements for estimating intraclass correlations with desired precision.” Statistics in Medicine, 21, 1331–1335. doi:10.1002/sim.1108 .