A Majorly Unique Occupational Cohort
Occupational epidemiology seeks to identify hazardous exposures in the workplace using various methods of research. The gold standard for such research is the occupational cohort study, in which workers are enrolled and followed throughout their career in an industry and beyond, if possible.
Some of the major challenges for any cohort study are selection bias and incomplete follow-up. This is indeed a poignant issue in occupational epidemiology as incomplete record keeping can make cohort enumeration difficult and workers can easily be lost when they leave employment.
An ideal occupational cohort would have several key characteristics:
1. A complete listing of all the workers who ever worked in the industry (at least during a substantially long study period);
2. Complete work-history information for each of these individuals, including hire and separation dates and time spent at various positions;
3. Complete vital-status for all members throughout the study period;
4. Extensive accrued follow-up time spanning a long period;
5. A solid and reliable exposure measure;
6. A low-cost to collect, process, and analyze the data
Seldom does something real come close to an ideal, but for once we have one: Major League Baseball (MLB).
Examining MLB through the lens of the criteria listed above, we see the following:
1. A complete listing of all the workers who were ever in the industry.
All Major League players are represented in the data, no matter how much or how little they played. There are numerous records of players who, in the last game of the season, got to bat one time, then never played again.
2. Complete work-history information for each of these individuals, including hire and separation dates and time spent at various positions.
Data on Major League Baseball has complete seasonal performance information dating back to 1871.
3. Complete vital-status for all members throughout the history of MLB.
The Society of American Baseball Research has a Biographical Research Committee, staffed by over 100 volunteers, whose sole mission it is to maintain accurate biographical data on Major League ballplayers, including date and place of birth and death. The committee researches player deaths using the Social Security Death Index, news media reports, and contact with surviving family members.
4. Extensive accrued follow-up time spanning a long period.
Between 1900 and 1999 alone, Major League Baseball players accrued over 311,000 person-years of observation.
5. A solid and reliable exposure measure.
While there have been slight changes in the rules of the game, the basic physical requirements and duties of the player positions have not changed in at least 100 years. Number of games played, number of plate appearances, and number of innings pitched are all reliable measures of exposure.
6. Be low-cost to collect, process, and analyze.
Though perhaps not truly low cost to collect, Baseball data are collected as a matter of course by Major League Baseball (including biographical data) and are made available on several websites, depending on the sorts of information and level of detail one seeks. These sources offer free downloads.
Now of course no cohort is perfect, and MLB data does have some serious weaknesses to match its strengths.
One of the biggest concerns from an epidemiological perspective is that once players leave the Major Leagues, their occupations are generally unknown, save for those players who go on to coach or manage in the Majors. A related point is that in the late 1800s and early 1900s baseball players were not yet true professionals – that is to say, almost all of them had off-season jobs. This opens the cohort up to untold exposures that may dramatically alter the effects on health outcomes of having been a professional athlete. Similarly, as only mortality and not morbidity data are known, there could be serious confounding from other medical conditions that are impacting survival estimates.
In spite of these problems, many studies have been published using data from Major League Baseball. A future post will review and explore some of these studies.