Rating evidence by the product rule

Kenneth A. Rahn
Draft, April 2001 (in progress)

    Lately I have been thinking once more about how great majority of the fights over evidence on the JFK assassination are useless, where "useless" is defined as never seeming to resolve the questions. As I wondered why I considered them useless, I realized that it was because the evidence in question was either indirect (related to something other than the point in question), untestable (of a type whose truth or falsity could not be determined), or both. Although there are other equivalent ways to express these properties of evidence, I find the concepts of directness and testability to be among the simplest and most informative.
    The key point here is that in order for evidence on a point to be important (truly useful), it must be both direct and testable If it lacks either property, it is unimportant. For example, a piece of evidence may be direct but unimportant because it is not testable. Alternately, a piece of evidence may be testable but unimportant because because it is indirect.
    "Importance" of evidence can also be expressed mathematically as the product of directness and testability:

Importance = Directness x Testability

    If we assign values of 0 and 1 to the variables (0 = indirect, untestable, or unimportant; 1 = direct, testable, or important), we can show the four products (combinations) in the table below:

    This might be called the "product rule." Note how there are three ways for evidence to be unimportant and only one way for it to be important. In other words, it is easy for evidence on a particular point in question to be unimportant. This extremely important, simple principle is often forgotten when considering evidence from the JFK assassination. It amounts to saying that the default case for evidence is unimportance rather than importance.
    Of course, actual evidence will more complicated than this because both directness and testability can have values that fall between 0 and 1. Thus, actual importances might be calculated as (0.25)(0.5) = 0.125, (0.1)(1) = 0.1, (0.9)(0.5) = 0.45, (0.9)(0.1) = 0.09, and so on. We can get a more general sense of this multiplicative relation by plotting the product against the two variables, as shown below:

    Note how more than three-quarters of the area of the plot has importances of ≤0.25.Also note how small is the zone in the upper right that has importances of ≥0.75. Again the message is how easy it is for evidence to be unimportant.

Examples
    Some examples can perhaps clarify the point. The table below shows how several types of evidence currently being discussed on the newsgroups alt.conspiracy.jfk and alt.assassination.jfk can be categorized, with brief reasons noted.