Engineering Misconceptions, Part 1 - Return Periods

Wrong: The 50-year return period wind speed of 55 m/s occurs once every 50 years.

Wrong: A wind speed of 60 m/s has been recorded recently in this area. Therefore the 55 m/s 50-year return period wind speed is now wrong.*

Right: The 50-year return period wind speed of 55 m/s has a 63% chance or probability of occuring or being exceeded at least once in the next 50 years.

Facts:
A 50-year return period event (say wind storm or earthquake) is also called an event with a 50-year recurrence interval, or a 1-in-50 year event.

This all actually means it has a 1/50 (2%) probability or chance of occurring in 1 year.

But, as mentioned above, it only has a 63% chance of occurring in the next 50 years.

It actually has a 37% chance of NOT occuring in the next 50 years.

Therefore, no one can say it WILL happen ONCE in a span of 50 years.

It CAN happen ONCE in 50 years.

It also CAN not-happen ONCE in 50 years.

It also CAN happen MORE THAN ONCE in 50 years.

It CAN happen tomorrow.

It CAN not-happen at all.

The probability is based on STATISTICAL ANALYSIS, based on the available data and data quality, and based on the modelling approach used for analysis.

It is a PROBABILITY and not a PREDICTION.

*To be discussed further in the next Engineering Misconceptions