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Introduction Research suggests that there is often a high degree of missingness in youth body mass index (BMI) data derived from self-reported measures, which may have a large effect on research findings. The first step in handling missing data is to examine the levels and patterns of missingness. However, previous studies examiningyouth BMI missingness used logistic regression, which is limited in its ability to discern subgroups or identify a hierarchy of importance for variables, aspects that may go a long way in helping understand missing data patterns. Methods This study used sex-stratified classification and regression tree (CART) models to examine missingness in height, body mass and BMI data among 74 501 youth participating in the 2018/19 COMPASS study (a prospective cohort study examining health behaviours among Canadian youth), where 31% of BMI data were missing. Diet, movement, academic, mental health and substance use variables were examined for associations with missingness in height, body mass and BMI. Results CART models indicated that the combination of being younger, having a selfperception of being overweight, being less physically active and having poorer mental health yielded female and male subgroups highly likely to be missing BMI values. Survey respondents who did not perceive themselves as overweight and who were older were unlikely to be missing BMI values. Conclusion The subgroups identified by the CART models indicate that a sample that deletes cases with missing BMI would be biased towards physically, emotionally and mentally healthier youth. Given the ability of CART models to identify these subgroups and a hierarchy of variable importance, they are an invaluable tool for examining missing data patterns and appropriate handling of missing data.

Additional file 1.

Supplemental material, sj-pdf-1-msc-10.1177_0969141320941900 for Perceived susceptibility to developing cancer and screening for colorectal and prostate cancer: A longitudinal analysis of Alberta’s Tomorrow Project by Meghan Gilfoyle, Ashok Chaurasia, John Garcia and Mark Oremus in Journal of Medical Screening

Supplemental material, sj-pdf-1-msc-10.1177_0969141320941900 for Perceived susceptibility to developing cancer and screening for colorectal and prostate cancer: A longitudinal analysis of Alberta’s Tomorrow Project by Meghan Gilfoyle, Ashok Chaurasia, John Garcia and Mark Oremus in Journal of Medical Screening

Changes in the number of types of beverages available in vending machines within participating COMPASS secondary schools (n = 78) within three policy groups: Alberta (n = 9), Ontario – Public (n = 64), and Ontario – Private (n = 5). (DOCX 53 kb)

Changes in the number of types of beverages available in vending machines within participating COMPASS secondary schools (n = 78) within three policy groups: Alberta (n = 9), Ontario – Public (n = 64), and Ontario – Private (n = 5). (DOCX 53 kb)