This is a SKAI website examining the bias of Observational Methods of causal inference using Randomized Controlled Trials with Imperfect Compliance.
This website contains:
The basic question we are asking is whether Observational Methods of causal inference offer biased estimates of causal effects, how large is this bias and how it is distributed. Observational Methods of causal inference consist in inferring the effect of a treatment by comparing treated and untreated units that have similar observed characteristics. Observational Methods of causal inference try to solve the bias due to confounding factors (a.k.a. selection bias) by accounting for the bias due to observed confounders. The hope is that the bias due to unobserved confounders is small enough to be negligible.
The main advantage of Observational Methods over the other methods of causal inference (Randomized Controlled Trials and Natural Experiments) is their broad applicability: they can almost always be implemented to estimate a treatment effect. With Observational Methods, we can eschew the difficulties of implementing a Randomized Controlled Trial in the field. We also can apply Observational Methods even when no clear Natural Experiment has been identified in the data.
The key question is whether the ease-of-use of Observational Methods comes at a huge cost in terms of bias. This is this question that we examine here.
Consider a policymaker deciding whether to expand a social program. Suppose she has access to observational data that records outcomes for users and non-users, allowing her to form an observational estimate of the program’s impact. Her immediate concern is selection bias: differences in outcomes might reflect differences in who chooses to take up the program rather than the causal effect of program participation. Because of these concerns, many advisors would argue that she should spend additional resources to conduct a Randomized Control Trial (RCT), gathering experimental data that allows unbiased estimation of the program’s effects. What should she do?
The extent of selection bias in the observational study determines whether she should run the RCT. If she knew the bias was negligible, there would be no reason to do so. Similarly, if there were a known positive bias (i.e., a bias that favours program adoption), or a known negative bias, she could simply adjust her observational estimate. The harder case, and that which the advisors would highlight, is when the sign and magnitude of bias are uncertain. If she knew its distribution, she could adjust her confidence in the observational estimate, and make a decision on that basis. But without further information on the distribution of bias, she must simply take a guess at the best option.
Our goal is to take the guesswork out of this decision. We treat selection bias as a parameter to be estimated, and the numerous imperfect compliance RCTs run in the last 20-30 years as a large untapped data source.