outcome (third interpretation above). In other words, most health outcomes of interest have multifactorial etiologies.

A good example is coronary heart disease (CHD). It has been amply demonstrated that smoking, high blood pressure, lack of exercise, and high serum cholesterol levels are all causally related to the development of CHD. Nonetheless, many people with one or more of these risk factors do not develop CHD, and some cases of CHD occur in people without any of the risk factors. Most of the adverse events considered by the committee have multifactorial etiologies.

The causal relation between a vaccine and a given adverse event can be considered in terms of three different questions (Kramer and Lane, 1992):

1. Can It? (potential causality): Can the vaccine cause the adverse event, at least in certain people under certain circumstances?

2. Did It? ("retrodictive" causality): Given an individual who has received the vaccine and developed the adverse event, was the event caused by the vaccine?

3. Will It? (predictive causality): Will the next person who receives the vaccine experience the adverse event because of the vaccine? Or equivalently: How frequently will vaccine recipients experience the adverse event as a result of the vaccine?

Each of these causality questions has a somewhat different meaning, and for each, there are different methods of assessment. In the section below, each question will be discussed in turn, with reference to how it relates to the committee's charge and how the committee attempted to answer it.

The committee has been charged with answering the Can It? causality question for the relations between vaccines routinely administered to children and several specific adverse events. The question is conventionally approached through controlled epidemiologic studies. (The term epidemiologic studies is used throughout this report in its broad sense to denote studies of disease and other health-related phenomena in groups of human subjects. The term thus includes many clinical studies but excludes animal and in vitro studies on the one hand and individual case reports on the other. See below the section Sources of Evidence for Causality for a more detailed description of epidemiologic studies.) Can It? is generally answered in the affirmative if the relative risk (the ratio of the rate of occurrence of the adverse event in vaccinated persons to the rate in otherwise comparable