Quantitative/ Analysis
ANCOVA
Compare outcomes after defensible covariate adjustment
ANCOVA is a linear model combining group indicators with continuous covariates. In a randomised pre-test and post-test study, modelling follow-up with baseline adjustment can improve precision. Covariates require substantive justification and suitable functional forms. The traditional common-slope version assumes no group-by-covariate interaction, which should not be imposed blindly.
WHEN IT FITS
Use for continuous follow-up outcomes with informative baseline measurements or justified pre-exposure covariates. Specify adjustment before analysis and avoid conditioning on variables produced by the treatment.
Strengths
- Can improve precision by using baseline information
- Expresses adjusted differences in outcome units
Limitations
- Post-treatment adjustment can introduce bias
- Common slopes and extrapolated adjusted means can mislead
Know the boundary
ANCOVA adjustment does not make an observational comparison equivalent to randomisation.
USED ACROSS
Banking & financeBusiness & MBAPsychologyComputer scienceEducation