Emphasizing the measurement of causal effects to arrive at a better understanding of the causal mechanisms involved in statistical theory, a mathematical model for causal inferences in prospective studies is developed and then applied to retrospective case-control studies. Before developing the model, causal agents are delineated, and causal effects are distinguished from "gains over time". The formal model is presented considering indirect measurement of causal effects, homogeneous populations, intermediate-level causal effects, the selection variable, randomization and the role of covariates. In the retrospective case-control studies, retrospective and prospective probabilities and matching are discussed. A loglinear model for a case-control study problem is presented. (Author/CM)