In this manual, the preferred terminology for describing the probability of exceedance is annual exceedance probability (AEP).

There are several ways to express AEP. The TxDOT preferred unit for expressing AEP is percent. An event having a 1 in 100 chance of occurring in any single year will be described in this manual as the 1% AEP event. Annual recurrence interval (ARI), or return period, is also used by designers to express probability of exceedance. A 5-year return interval is the average number of years between years containing one or more events exceeding the specified AEP. Lastly, AEP can also be expressed as probability (a number between 0 and 1), such as p = 0.01. Examples of equivalent expressions for exceedance probability for a range of AEPs are provided in Table 4-1.

Table 4-1: Three Ways to Describe Probability of Exceedance

AEP (as percent)	AEP (as probability)	Annual Recurrence Interval (ARI)
50%	0.50	2-year
20%	0.20	5-year
10%	0.10	10-year
4%	0.04	25-year
2%	0.02	50-year
1%	0.01	100-year

While AEP, expressed as a percent, is the preferred method for expressing probability of exceedance, there are instances in this manual where other terms, such as those in Table 4-1, are used. These instances include equation subscripts based on return period

(e.g. Q

10

)

, plot axes generated by statistical software, and text and tables where readability was improved as a result.