There were considerable differences in the potential for internal radiation exposure and the monitoring policies that were followed at each facility. For this reason internal radiation exposure was represented as a factor (that is referred to as IG) with three levels:
The precise definition of this variable was facility dependent and changed over time--see [2,3]. Exposure analysis files that contained yearly values for external dose and the internal exposure indicator variable for each individual at each Oak Ridge facility were created (see Appendix). In the dose-response analyses radiation exposure is a time dependent variable, and cumulative dose (with a lag) was used to form dose groups for external radiation. For internal exposure an individual was assigned to the appropriate category based on the sequential yearly values for the internal exposure indicator variable.