Table 1.

Collider stratification bias scenario simulation input parameters and resulting association between obesity and mortality, illustrated for renal cancer patients

Normal weightOverweightObese
Input parameters based on published data
 Prevalence of weight status in the populationa0.3330.3330.333
 Lifetime cancer riskb0.0120.0160.021
 Effect of weight status on cancer incidencecRefOR = 1.50OR = 2.00
Input parameters based on expert opinion
 Effect of unmeasured risk factor A on cancer riskdOR = 1.50OR = 1.00OR = 1.00
 Effect of A on death of cancer patientsdOR = 2.00OR = 2.00OR = 2.00
Causal compared with resulting association between weight status and mortality among renal cancer patients
 Causal OR for weight status and mortality among cancer patientseRefOR = 1.10OR = 1.30
 Resulting OR for weight status and mortality among cancer patientsRefOR = 0.86OR = 1.00
  • NOTE: We assumed 5-year survival among cancer-free people of 0.93 (estimated based on U.S. life tables for 65-year-olds; ref. 15) and 5-year survival among cancer patients of 0.74 (based on SEER statistics; ref. 16). Results based on 1,000 replications.

  • aApproximate prevalence of overweight and obesity in U.S. adult population (17). Conceptually, our simulations assume that each individual's weight status is stable throughout the period they are at risk for developing renal cancer.

  • bLifetime cancer risk estimates by weight status are estimated to be consistent with SEER report of lifetime risk of kidney and renal pelvic cancers (0.016; ref. 16).

  • cEstimate of effects of weight status on renal cancer incidence from a large U.S. cohort (18).

  • dDistribution of unmeasured risk factor “A” mean = 0.0 and SD = 1.0 for all weight status groups. “A” may represent a single risk factor or the combined effects of multiple risk factors operating in concert; examples of possible “A” variables include an unmeasured genetic risk variant, lifetime exposure to cigarette smoking, or a harmful occupational exposure.

  • eThe causal OR for weight status and mortality among cancer patients assumes that the true causal OR for weight status on mortality is the same in cancer patients as in the general population of U.S. adults. (ORs of 1.10 and 1.30 for overweight and obesity respectively; ref. 19).