Dear Professor Mean, I review a lot of observational studies in the literature, and I am concerned about the response rates and when they fall so low that they tend to produce problems with selection bias. I’ve heard that anything lower than 80% is a problem. Is that correct? Continue reading
Monthly Archives: March 2015
PMean: Nonparametric tests for multifactor designs
Dear Professor Mean, I want to run nonparametric tests like the Kruskal-Wallis test and the Friedman test for a setting where there may be more than one factor. Everything I’ve seen for these two tests only works for a single factor. Is there any extension of these tests that I could use when I suspect that my data is not normally distributed. Continue reading
PMean: Forget confounding, and think of things in terms of covariate imbalance
Someone noted in a passing comment in their email that they found the term “confounding” to be difficult and confusing. I’ve been doing this stuff for over thirty years, but to be quite honest, I get a little nervous about this as well. But I took the time to explain a simpler concept, “covariate imbalance.” Continue reading