LTAS.NET: A NIOSH Life Table Analysis System for the windows environment

Purpose Life table analysis is a fundamental tool of occupational epidemiology. A life table analysis system (LTAS) was developed by the National Institute for Occupational Safety and Health (NIOSH) in the 1980s. The current system, called PC-LTAS, is limited by its platform (MS-DOS) and by its analysis and reporting capabilities. A project was initiated to create a LTAS for the Windows operating system (LTAS.NET) that would permit the analysis of more than one exposure variable, as well as allow stratification by user-defined fixed and time-dependent covariates. Methods A group of epidemiologists, programmers and statisticians developed system and analysis requirements. The LTAS.NET program is written in Microsoft Visual Studio.NET using a SQL Server database engine. Statistical methods include the use of (indirectly) standardized mortality ratios, (directly) standardized rate ratios, confidence intervals based on Poisson and exact methods, and the Rothman trend test for analyses of linear exposure-response associations. Comprehensive software testing strategies (including algorithms for person-time stratification and statistical calculations) were employed in the development of LTAS.NET. Results The LTAS.NET program allows for simultaneous stratification and analysis of multiple exposure variables. Time-dependent and fixed user-defined variables, and globally defined temporal variables, can be incorporated. The import, stratification and results reporting options are highly flexible. LTAS.NET supports the use of exposure lags and consideration of active and inactive (working) person-time. Users may export stratified event and person-time data for use in Poisson regression modeling software. Conclusion The NIOSH LTAS.NET incorporates a number of methodological improvements that should facilitate more complex life table analysis of occupational cohort data than was possible in PC-LTAS. NIOSH plans to release LTAS.NET to the public in the future.