Probability:

If E₁ is the number of heads, and E₂ is the number of tails, E₁/(E₁  + E₂) is an experimental determination of the probability of heads resulting while a coin is flipped.

P(E_l) = n/N

As definition, the probability of an event must be greater than or equal to 0, and less than or equal to l.  Also, the sum of the probabilities of all outcomes over the whole "event" must add to equal l. For instance, the probability of heads in a flip of a coin is 50%, the probability of tails is 50%.  If we suppose these are the only two probable outcomes, 50% + 50%, the two outcomes, equals 100%, or 1.

The concept of probability is used in statistics while considering the reliability of the data or the measuring device, or in the correctness of a decision.  To have confidence in the values measured or decisions made, one must have an assurance in which the probability is high of the measurement being true, or the decision being correct.

To compute the probability of an event, the number of successes (s), and failures (f), must be determined.  Once this is determined, the probability of the success can be computed by:

P = s/(s+ f)

where

s + f = n = number of tries.