Regression Discontinuity Misuse — When Logic Wears a Disguise
Errors arising from improper application of regression discontinuity designs, including incorrect functional form assumptions, inappropriate bandwidth selection, or failure to detect manipulation of the assignment variable near the cutoff. These errors can produce spurious treatment effects.
Also known as: RDD Misspecification
How It Works
Regression discontinuity is seen as a quasi-experimental 'gold standard,' and its technical requirements are easily overlooked. The apparent rigor of the design provides false confidence.
A Classic Example
A study of a scholarship program uses a test score cutoff but fits a linear model to data that is actually curved, producing a false 'jump' at the cutoff that is really a modeling artifact.
More Examples
A city evaluates a speed camera program by comparing accident rates just above and below the 30 mph speed limit used to determine camera placement. Researchers use a very narrow bandwidth of only a few observations near the cutoff, producing an unstable estimate that swings wildly with the inclusion or exclusion of a single data point, and falsely conclude the cameras have no effect.
A study assessing whether students who barely pass a remedial math course (cutoff: 60%) earn higher wages later in life fits a single straight regression line across a dataset where the true relationship curves sharply near the cutoff. The apparent 'jump' in wages at the threshold is largely an artifact of forcing a linear model onto nonlinear data, not evidence that the course itself raises earnings.
Where You See This in the Wild
Education policy evaluation, electoral threshold studies, regulatory compliance analysis, and health economics.
How to Spot and Counter It
Test sensitivity to bandwidth and functional form choices. Check for bunching at the cutoff that would indicate manipulation. Use robust estimation methods.
The Takeaway
The Regression Discontinuity Misuse is one of those reasoning errors that sounds perfectly logical at first glance. That's what makes it dangerous — it wears the costume of valid reasoning while smuggling in a broken conclusion. The best defense? Slow down and ask: does this conclusion actually follow from these premises, or am I just connecting dots that happen to be near each other?
Next time someone presents you with an argument that "just makes sense," check the structure. The feeling of logic is not the same as logic itself.