The probability of an event could be inferred from the frequency ratio definition given above. On the other hand we could apriori obtain the probability of an event, without performing any experiment, based on certain symmetry considerations. For example one could predict before hand that the probability of getting a head in tosses of a coin would be 0.5, purely from symmetry. Similarly the probability getting a particular face of a six faced die would be 1/6. In Statistical Mechanics it is assumed that different micro states have equal probability.

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