Using Bounds to Compare the Strength of Exchangeability Assumptions for Internal and External Validity.

We describe bounds for the risk difference of a binary exposure on a binary outcome under four common study settings: observational and randomized studies, each with and without simple random selection from the target population. Through these scenarios, we introduce randomizations for selection and treatment, and the width of the bounds are narrowed from 2 (the width of the range of the risk difference) down to 0 (point identification). We then assess the strength of the assumptions of exchangeability for internal and external validity by comparing their contributions to the width of the bounds in the setting of an observational study without random selection from the target population. We find that when less than 1/2 of the target population is selected into the study, the assumption of exchangeability for external validity of the risk difference is stronger than that for internal validity. The relative strength of these assumptions should be considered when designing, analyzing, and interpreting observational studies, and will aid in determining the best methods for estimating the causal effects of interest.
PMID: 30834430 [PubMed - as supplied by publisher]

Source: Am J Epidemiol - March 4, 2019 Category: Epidemiology Authors: Breskin A, Westreich D, Cole SR, Edwards JK Tags: Am J Epidemiol Source Type: research