The Communications, Energy and Paperworkers Union has blasted the report. On the difficulty of measuring the economic output today, compared to 40 years ago. The probability of an event is the long run frequency of its occurrence. Two events are independent if the occurrence of one does not make the occurrence of the other more or less probable. If two events are independent, the probability of both of them occurring can be calculated by multiplying.
Events are often dependent.
When events are dependent, the probability of one event, say A, given that the other event, say B, has occurred, is now the frequency of occurences of A among situations where B has occurred. This is usually a small, even tiny, probability. Here's a cute example from The prosecutor , attributed to Keynes. Suppose you are playing poker against the Archbishop of Canterbury and that on the first hand he deals himself a straight flush. Most people would have a rather large probability as an answer to the second question though.