SEER*Stat prevalence calculations use the Counting Method. Cases still alive on the desired prevalence date are counted, while adjustments are made to estimate the proportion of cases lost to follow-up that would have made it to the prevalence date. The expected number of cases lost to follow-up who make it to the prevalence date is computed using conditional survival curves for specified cohorts. The current version of SEER*Stat uses observed survival. The survival calculations for prevalence use the Kaplan-Meier method with monthly intervals. Future versions will support an alternate approach which utilizes expected survival and therefore will account for differences in survival based on expected variables such as age, race, sex, and year.

Use the Survival Cohorts tab to define survival cohorts that are appropriate for your analysis. It is vital that these cohorts are defined with a level of detail to account for differences in survival, but not so small that the survival statistics are unstable. This must be done with care. Since observed survival is being used, it is recommended that age and year be included as cohort variables in most analyses. The combinations for the variables, based on groupings, define the cohorts. Each lost case must fit into one and only one cohort; therefore, variables with overlapping groupings are not allowed and the cohorts cannot exclude any records included in the analysis. That is, only cases that meet the criteria on the Selection tab are used in the survival calculations.

Prevalence statistics released by the Surveillance Research Program use age at diagnosis (< 60, 60-69, 70+), year of diagnosis (5-year periods), sex, cancer site, and race as survival cohort variables (see Considerations for Survival Cohort Definitions).

In a future version of SEER*Stat, you will be able to choose whether to use the Observed Survival Method or the Profile Method in computing survival for cases lost to follow-up. However, the Profile Method is not available at this time.

To define survival cohorts, select one or more of the Available Variables and click Add to make them Cohort Variables. (The layout of the Available Variables box is described in Variable Categories. Use the Find button if you are having trouble locating a variable.) The order of the cohort variables does not affect the calculations, but does determine the layout of the survival tables that are generated. To change the order, highlight a variable and click Move Up or Move Down. Use the Remove button to remove an unwanted cohort variable.

Note: Calculated variables are not available on the Survival Cohorts tab.