What is collider bias?

A paper by Holmberg et al. (2022) in JAMA provides a number of examples of how collider bias can lead to problematic causal inference. The term collider bias is often invoked when using directed acyclic graphs (DAGs) to map the causal pathway. Collider bias occurs when you aim to measure the impact of A on…

I have too many control variables…which ones should I include in my regression model?

Supposed you have some data on health care spending for different individuals and you want to know which patient characteristics increase health care spending. While this seems like something any health economist could do, measuring the relationship require both knowing (i) which independent variables to include in your data analysis and (ii) their functional form.…

Distributional Cost Effectiveness Analysis: A simple example

I have written previously about the need for distributional cost-effectiveness analysis (DCEA) which would increase the value of treatments who improve the health for individuals who currently have the worst expected quality-adjusted life expectancy (QALE). I’ve written about this in AJMC and Health Affairs among other places. One key question many individuals may have is…

Should you adjust for covariates when analyzing data from randomized controlled trials?

FDA draft guidance published this month says you should. In most cases, adjusting for covariates is not necessary. Randomization generally insurers that covariates are balanced across clinical trial arms. Randomization, however, may not always result in perfectly balanced trial arms. In these cases, the FDA notes that covariate adjustment is perfectly acceptable. There are some…

What is immortal time bias?

A recent JAMA paper by Yadav and Lewis (2021) provide the answer: Bias from immortal time periods is the error in estimating the association between the exposure and the outcome that results from misclassification or exclusion of time intervals Yadav and Lewis (2021) Sounds simple but how do these errors occur in practice? Here are…