Why measures of variability matter: Average age of death in The Olden Days

Alright, this is a 30-second long example for a) bimodal distributions and b) why measures of variability matter when we are trying to understand a mean.

And that mean is...AGE OF DEATH.

My inspiration for this tweet is:

I’m just a girl, standing in front of the internet, asking it to understand that historical life expectancies doesn’t mean most people died at 45 but rather that infant mortality was super high and pulled down the average.

— Angelle Haney Gullett (@CityofAngelle) January 12, 2022

Gullett refers here to the commonly held belief that if the mean life span Back In The Day was 45, or thereabout, everyone was dying around 45. NOT SO.

Why?

The short answer is no. Broadly speaking, there were two choke point of human mortality. Younger than 5, and again around 50. If you made it through those, barring accidents, you likely had what was a normal lifespan of ~65-70 years.

And this is why I’m no fun at parties 😂

— Angelle Haney Gullett (@CityofAngelle) January 12, 2022

OK. Angelle is invited to my parties. She would tell us how many humans didn't survive childhood. AND THEN...a bunch of people would croak around 50. 

*Let's take a moment and send some good thoughts and gratitude to the memories of Jonas Salk, Edward Jenner, and Alexander Flemming.*

Yet another Twitterer then shared this data visualization to demonstrate how mortality has changed over time. 

Here is the data on that.

Yes, it was was extremely common that children died, as you say.

But that was not the only reason for low life expectancy. Mortality at higher ages was also very high. Much higher than today.

And so it was the case that exactly half were dead by 45. pic.twitter.com/ZK0REZQR57

— Max Roser (@MaxCRoser) January 14, 2022

When I shared this in class, I threw these Tweets up on a PPT slide. Again, it shows a roughly bimodal distribution, with death increasing in childhood and 50. It also demonstrates why we need to discuss the variability in a data set, not just the midpoint.