R

In an earlier post, I described in a fair amount of detail an algorithm to generate correlated binary or Poisson data. I mentioned that I would be updating simstudy with functions that would make generating these kind of data relatively painless. Well, I have managed to do that, and the updated package (version 0.1.3) is available for download from CRAN. There are now two additional functions to facilitate the generation of correlated data from binomial, poisson, gamma, and uniform distributions: genCorGen and addCorGen. Here’s a brief intro to these functions.

Ideally, a study that uses randomization provides a balance of characteristics that might be associated with the outcome being studied. This way, we can be more confident that any differences in outcomes between the groups are due to the group assignments and not to differences in characteristics. Unfortunately, randomization does not guarantee balance, especially with smaller sample sizes. If we want to be certain that groups are balanced with respect to a particular characteristic, we need to do something like stratified randomization.

Using the simstudy package, it’s possible to generate correlated data from a normal distribution using the function genCorData. I’ve wanted to extend the functionality so that we can generate correlated data from other sorts of distributions; I thought it would be a good idea to begin with binary and Poisson distributed data, since those come up so frequently in my work. simstudy can already accommodate more general correlated data, but only in the context of a random effects data generation process. This might not be what we want, particularly if we are interested in explicitly generating data to explore marginal models (such as a GEE model) rather than a conditional random effects model (a topic I explored in my previous discussion). The extension can quite easily be done using copulas.

Say we have an intervention that is assigned at a group or cluster level but the outcome is measured at an individual level (e.g. students in different schools, eyes on different individuals). And, say this outcome is binary; that is, something happens, or it doesn’t. (This is important, because none of this is true if the outcome is continuous and close to normally distributed.) If we want to measure the effect of the intervention - perhaps the risk difference, risk ratio, or odds ratio - it can really matter if we are interested in the marginal effect or the conditional effect, because they likely won’t be the same.