Illustrating Poststratification Methods in Medical Claims Data: A Korean Example

Background: Observational studies often rely on nonprobability samples that may not represent the target population, limiting the generalizability of findings. Health examination data from the Korean National Health Insurance Service (NHIS) faces this challenge, as voluntary participation introduces sampling bias. Poststratification offers a potential solution by reweighting samples to match population distributions. Methods: Using the NHIS-National Sample Cohort (NHIS-NSC), we demonstrated three poststratification approaches—simple poststratification, raking, and multilevel regression with poststratification—to estimate obesity prevalence among adults aged 20–39 years in 2019. The Korea National Health and Nutrition Examination Survey (KNHANES) served as the reference standard for population-level estimates. We compared these methods with inverse probability sampling weights. Additionally, we applied poststratification to evaluate the accuracy of self-reported disease history for hypertension, diabetes, dyslipidemia, and stroke. Results: Crude obesity prevalence from NHIS-NSC was 36.3% (95% CI: 36.0–36.6), substantially higher than the KNHANES reference of 31.4% (28.8–33.9). Simple poststratification using age and sex reduced this estimate to 33.9% (33.6–34.2), raking with additional smoking and alcohol variables yielded 34.7% (33.9–35.5), and multilevel regression with poststratification incorporating region produced 34.2% (33.3–35.0). inverse probability sampling weights yielded comparable results (33.9–34.1%). For self-reported disease history, poststratification consistently produced modest decreases in sensitivity estimates, suggesting that health examination participants may report disease history more accurately than the general population. Conclusions: Poststratification provides a principled framework for improving population-level inferences from non-probability samples. These methods warrant broader application in epidemiological research using administrative and electronic health record data.