How Much Overlap is Too Much?

There have been a couple of blog and backchannel questions about the acceptable degree of overlap between previous homework assignments and the final project.

The general rule is that as long as 51% or more of the model has changed, you are OK. That is,

1. if you are using some of the same variables, but in a different model (i.e., the homework model was an additive hierarchical model and now you are looking at the same variables in a mediated or moderated model), that's OK. (Note that the converse is not true--a mediated or moderated homework model cannot revert to an additive hierarchical model, because in that case, you would have no need to run the regressions, because they were already done in the original homework assignment. That is, regression 3 from a mediated model is exactly what you would run to test an additive model with those variables. Similarly, step 1 of a moderated model is the additive multiple regression model.)

1. if the outcome is the same, but predictor and mediator are different, that’s OK.

2. if one predictor is the same, but a moderator, mediator, or second predictor have been added, and the outcome is different, that's OK.

3. if a mediator is the same, but the predictor and outcome are different, that's OK.

If you aren't sure if your model fits those parameters, feel free to ask, but have patience for the reply. I start jury duty tomorrow and will not have internet access...