Sample size for a Pearson correlation confidence interval using a planning value from a prior study
Source:R/statpsych2.R
size.ci.cor.prior.RdComputes the sample size required to estimate a Pearson correlation with desired confidence interval precision in applications where an estimated Pearson correlation from a prior study is available. The actual confidence interval width in the planned study will depend on the value of the estimated correlation in the planned study. An estimated correlation from a prior study can be used to compute a prediction interval for the value of the estimated correlation in the planned study. If the prediction interval includes 0, then the correlation planning value is set to 0; otherwise, the correlation planning value is set to the lower prediction limit (if the prior correlation is positive) or the upper prediction limit (if the prior correlation is negative). Using a larger confidence level (1 - alpha2) for the prediction interval will increase the probability that the width of the confidence interval in the planned study will be less than or equal to the desired width.
This sample size approach assumes that the population Pearson correlation that was estimated in the prior study is very similar to the population Pearson correlation that will be estimated in the planned study. However, this type of prior information is typically not available and the researcher must use expert opinion to guess the value of the Pearson correlation that will be observed in the planned study. The size.ci.cor function uses a correlation planning value that is based on expert opinion regarding the likely value of the correlation estimate that will be observed in the planned study.