Tuesday, April 19, 2011

What is average and what does it mean?

I was met with frustration again this week when I was discussing something with a medical provider and she did not seem to understand some basic math terms that are used all of the time in the medical field. The specific point was the "median" life expectancy for those with Propionic Acidemia. Nothing has changed in the past 9.5 years and the median life expectancy remains at 3.5 years.

Median is the middle number when you are looking at a series. So in the series 1, 3, 5, 8, 9, 15, 17, 23, 49-- 9 is the median as it is the middle number. So to get 3.5 years as the median life expectancy in PA there are an equal number of affected individuals dying under age 3.5 years as those older than 3.5 years. Median is useful in describing a set of data or series where the numbers are in a far reaching range.

Mean is what people usually think of when they talk about averages, it is when you take a set of numbers and add them and then divide by how many numbers in the series. So looking at the same numbers above- 1, 3, 5, 8, 9, 15, 17, 23, 49-- Add them 1+3+5+8+9+15+17+23+49=130. Then divide 130 by 9 as there were 9 numbers in the series- 130/9=14.44

So with the same set of numbers the median and mean can be very different.

I was upset because this medical provider was telling me that propionic acidemia was not life threatening, when it is even when you are past the median life expectancy. She was essentially telling me that since she is older we don't have anything to worry about-- no, we do. We have to continue to be careful and cherish everyday because it is not known how long she could have. So shame on her for passing on misinformation from a person who should know these basic math concepts.

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