Using methods to extend inferences to specific target populations to improve the precision of subgroup analyses

Understanding the benefits and harms in specific subgroups of patients is an important part of epidemiologic and public health research. Unfortunately, commonly used methods to do subgroup analyses can result in estimates with lots of uncertainty. Repurposing methods that have traditionally been used to "generalize" or "transport" effect estimates from specific studies to the types of patients more likely to be encountered in the real world could be used to obtain more informative estimates in subgroups without ignoring differences between different types of patients. In this project, we applied this strategy to the Panitumumab Randomized Trial in Combination with Chemotherapy for Metastatic Colorectal Cancer to Determine Efficacy (PRIME) to create much less variable estimates of the treatment effect in Hispanic participants without ignoring the fact that there were more Hispanic participants with a tumor variation that changed the effect of treatment. On the other hand, when we tried to apply this strategy to improve estimates in patients with that tumor variation, we ended up with a misleading effect estimate. While these methods can reduce uncertainty about the benefits of treatment in specific subgroups interesting to researchers, they can result in incorrect subgroup estimates when their assumptions are violated.