An observational study is biased if the treated and control groups differ prior to treatment in
ways that matter for the outcomes under study. An overt bias is one that can be seen in the
data at hand-for instance, prior to treatment, treated subjects are observed to have lower
incomes than controls. A hidden bias is similar but cannot be seen because the required
information was not observed or recorded. Overt biases are controlled using adjustments,
such as matching or stratification. In other words, treated and control subjects may be seen …

An observational study is an empiric investigation of effects caused by treatments when
randomized experimentation is unethical or infeasible. The quality and strength of evidence
provided by an observational study is determined largely by its design. Excellent methods of
analysis will not salvage a poorly designed study. The line between design and analysis is
easier to draw in practice than it is in theory. In practice, the design of an observational study
consists of all activities that precede the examination or use of those outcome measures that …