Research in the probabilistic theory of information retrieval involves the construction of mathematical models based on statistical assumptions. One of the hazards inherent in this kind of theory construction is that the assumptions laid down maybe inconsmtent in unanticipated ways with the data to which they are applied. Another hazard is that the stated assumptions may not be those on which the derived modeling equations or resulting experiments are actually based. Both kinds of mistakes have been made m past research on probabihstic reformation retrieval. One consequence of these errors is that the statistical character of certain probabilistic IR models, including the so-called Binary Independence model, has been seriously misapprehended