Systematic patterns of zero exposures in event-history analysis.
Sociological Methodology, 30:1, 237–259 (2000)
Users of intensity-regression techniques in event-history analysis based on occurrence and exposure matrices frequently encounter zero-valued exposures where there is no observation for selected combinations of factor levels, either because they are logically impossible or because the observational design leaves them out. For example, higher educational levels cannot be attained at very young ages, and observations collected over a given calendar period cannot contain data for older birth cohorts at younger ages or data for younger cohorts at older ages. Unless the exposure matrix is too badly full of zeros, this does not necessarily cause problems for the estimation of regression parameters, but interpretation of results may have to be carried out with extra care. In this paper, we discuss why this is so and show how deft handling of systematic patterns of zero exposures can facilitate analysis. (BLACKWELL PUBLISHERS)