Wednesday, October 20, 2010

Simple Experiment Design Question

Consider the following hypothetical experiment to examine whether taking a headache pill reduces a headache. The 40 participants in the treatment group took a pill for their headache, and 50% of them have been cured. The 40 participants in the control group did not take the pill, and only 40% them have been cured. The results are in the table below. It seems clear that the pill helps reduce headaches.

Headache Gone Headache Not Gone % Cured
Pill 20 20 50
No Pill 16 24 40

However, suppose that this experiment did not properly randomize the assignment of subjects to the treatment or control groups. How can a confounding variable (e.g., the gender of the test subjects) invalidate the outcome above?


Panos Ipeirotis said...

Simpson's paradox?

Yisong Yue said...

Yes, it is indeed Simpson's paradox =)