To calculate SMOR, a mortality study can be thought as as a variant of a case-control study, in which cases are deaths from a specific cause and controls are deaths from other causes. The odds of "exposure" (in our study, the odds of having Down syndrome) are ascertained for both "cases" and "controls". This odds ratio can be standardized for covariates such as age and sex in the usual manner. We estimated the SMORs adjusted for age, sex, race, and death cohort.