We’re in an observational study setting in which treatment assignment was not controlled by the researcher. We have pre-treatment data on baseline outcomes and we’d like to incorporate them, mainly to decrease bias due to confounding and but also, ideally, to increase precision. One approach is to use the difference between pre and post outcomes as the outcome variable; another is to use the baseline data as a control. Which is better?
Mostly we use design diagnostics to assess issues that arise because of design decisions. But you can also use these tools to examine issues that arise after implementation. Here we look at risks from publication bias and illustrate two distinct types of upwards bias that arise from a “significance filter.” A journal for publishing null results might help, but the results in there are also likely to be biased, downwards.
We’ll be back on January 7 – Happy New Year!
In designs in which a treatment is assigned in clusters (e.g. classrooms), it’s usual practice to account for cluster-level correlations when you generate estimates of uncertainty about estimated effects. But units often share commonalities at higher levels, such as at a block level (e.g. schools). Sometimes you need to take account of this and sometimes you don’t. We show an instance of the usual procedure of clustering by assignment cluster (classrooms) working well and show how badly you can do with a more conservative approach (clustering by schools). We then show an example of a design in which clustering at the level of treatment assignment (classroom) is not good enough; in the troublesome example, schools are thought of as being sampled from a larger population of schools and treatment effects are different in different schools. In this case, if you want estimates of uncertainty for population level effects you have to cluster at the school level even though treatment is assigned within schools.
Imagine you are in the fortunate position of planning a collection of studies which you will later get to analyze together (looking at you metaketas). Each study estimates a site specific effect. You want to learn something about general effects. We work through design issues using a multi-study design with
J studies that employs both frequentist and Bayesian approaches to meta-analysis. In the designs that we diagnose these perform very similarly in terms of estimating sample and population average effects. But there are tradeoffs. The Bayesian model does better at estimating individual effects by separating out true heterogeneity from sampling error but can sometimes fare poorly at estimating prediction intervals.