Random changes are difficult to discuss, in that we cannot (usually) say that some specific change will happen, only that the change is probable or improbable. Statements involving probability are apt to produce discomfort in the reader, a vague sense of entering through a shop door labelled “only abstractions sold here”. This discomfort will be dispelled if the reader makes a practice of rewriting statements involving the word “probability” into rather more cumbersome statements about proportions. Suppose, for example, we read, “If a woman aged 46 reproduces, the probability that her child will suffer from Down’s syndrome is 1/25”. This means: if an indefinitely large number of women aged 46 reproduce, 1/25 of all the children born to such women will have Down’s syndrome.
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