1. Today’s topic: Does playing baseball shorten your life?
Vol. 2, Number 1, September 15, 2012
MV Stats News
Bringing multivariate data analysis and data visualization to your breakfast table
Today’s topic:
Does playing baseball shorten your life?
Michael Friendly, Staff Reporter
Filed: 7/21/2018 6:21 AM

2. Background
A National Institute for Occupational Safety and Health study (Mar., 2012) found that professional football players (NFL) lived longer, on average, than the general population
Surprising, but maybe not so much, since they are elite athletes and only the strongest, fastest and healthiest get to play
Notice that does NOT say that playing pro football will make you live longer--- probably quite the opposite!
And, is a comparison of pro football players with the general population at all reasonable or useful?
What other factors are not controlled?

3. Enter the sport journalist
Bill Barnwell decided to compare NFL players to baseball players, also elite athletes
  Baseball Football
Qualifying Players , ,088
Alive , ,694
Deceased
Mortality Rate % %
Apparently a highly significant association, p = 0.004!
library(vcd)
fourfold(tab)

4. Enter the sport statistician
But: there is a basic problem here:
Baseball players tend to be older than football players
MLB and NFL players are not directly comparable!
football
baseball
Technical note: this graph shows density estimates – optimally smoothed, much better than histograms

5. Models to the rescue!
Fit logistic regression models, predicting Alive (0/1) from dummy variable for Sport
Model1: Alive ~ Sport (ignoring age)
Model2: Alive ~ Sport + Age (controlling age)
Conclusion: Effect of sport disappears. It’s only the relative ages difference responsible for difference in mortality p=
p=0.444
p<2-16

6. Lessons
When you read “X causes Y,” the hairs on the back of your neck should stand up!
Careful writers report “association,” not causality
Always ask what has not been controlled or accounted for (age, sex, lifestyle, genetics,...)
Only randomized trials or experiments can claim causal explanations...
... but only if all other confounders have been controlled for.
References: Stats in the Wild blog, Aug. 23, 2012,