A common type of observational study compares population rates in several regions having differing policies in an effort to assess the effects of those policies. In many studies, particularly in public health and epidemiology, age-adjusted rates are regressed on predictor variables to give a covariance-adjusted estimate of effect; this estimate is shown to be generally biased for the appropriate regression coefficient. For familiar models, the analysis of crude rates with age as a covariate can lead to unbiased estimates, and therefore can be preferable. Several other regression methods are also considered.