Exceedance probability tells you the odds that a designated value is going to be exceeded. For example, if you have data regarding the average cost of bread over a 10-year span, exceedance probability calculations would allow you to determine the odds that bread will cost more than this average when you actually go to the store. Exceedance probability is used frequently in calculating the behavior of bodies of water. The reason for this is that it helps predict the probability that flooding might occur and therefore assists in the designs of bridges, dams and sewers.
Arrange the values that you have from greatest to least. For example, if you are trying to calculate exceedance probability for the peak flow of a river, you might have annual data regarding the water level. If this were the case, each year would most likely have a different peak flow level. In this situation, you would arrange peak flow levels from greatest to least.
Add up the total number of values. Using the example from Step 1, this would be equivalent to the number of years for which you have data since each year constitutes a single value.
Plug the total number of values into this formula: rank / (total number of values + 1). For example, if you found in the previous step that you had 9 values, you would insert this number into the formula: rank / (9 + 1).
Number each individual value, according to its rank in the arrangement. The greatest value should be ranked No. 1, the second greatest No. 2 and so on, until each value has a rank.
Decide which value you want to use as the standard. The standard value is the value for which you want to calculate the exceedance probability.
Take the rank of this standard value and plug it into the formula. For example, if you wanted to use the fifth largest value as the standard value in the calculation, you would plug the number "5" into the formula. This would yield 5 / (9 + 1).
Solve the formula. Using the example, you would calculate 5 / (9 + 1) = 5 / 10 = 0.50. Multiply the result by 100 so that the exceedance probability is expressed as a percent. In the example, you would multiply 0.5 by 100 to get 50 percent.
Data representing a longer period of time will result in more reliable calculations.