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What is Probability?

In common usage, the word "probability" means the possibility of some event.

It is a well-known fact now that the frequency of outcomes is a stable value in a long trial period. Many experimentalists tossed an unbiased coin and counted the frequency of "head" occurrences. Every time when the number of trials was great enough the frequency of "head" occurrences was about 0.5. For example, the next table shows the results obtained by French naturalist Buffon (1707-1788) in the XVIII century and results obtained at beginning of XX century by English statistician Pirson (1857-1936).

The effect described above and recurrent observations of other mass social and natural phenomena are the ground for the next conclusion. The frequency of event outcomes is nearly equal to some constant value in a long trial period. For example, the frequency of boys' birth is 0.518  nd the frequency of girls' birth is 0.482.

This constant value is called probability.