Exploring the external validity of survey data with triangulation: A case study from the Norwegian Offender Mental Health and Addiction (NorMA) Study

Objectives This paper demonstrates how to investigate the external validity of a study sample by triangulating survey and registry data, using data from the Norwegian Offender Mental Health and Addiction (NorMA) Study as a case. Methods We use survey data from the NorMA study (n = 1495), including the NorMA cohort (n = 733), and data from the Norwegian Prison Registry on all people imprisoned on 1 September 2013 (n = 3386). Triangulation was performed by (1) comparing the NorMA cohort to those lost to follow-up (n = 762), using survey data from the NorMA study. Secondly, we compared the NorMA cohort to the one-day population, using data from the Norwegian Prison Registry. We also stratified the one-day sample by possession of a Norwegian personal identification number (PIN). Results We found differences in birthplace, imprisonment and drug use between the NorMA cohort, lost to follow-up and the one-day population. Twenty-three percent of the one-day population did not have a Norwegian PIN. The NorMA cohort was more similar to those with a Norwegian PIN in the one-day population. Our triangulation demonstrates that 56–62% of the Norwegian prison population had an indication of drug use before imprisonment. Conclusions The NorMA cohort was overall representative of the one-day prison population holding a Norwegian PIN and less representative of prisoners without a Norwegian PIN. Using this method provides tangible inputs on the strengths and limitations of a study sample and can be a feasible method to investigate the external validity of survey data.