About historical minor league data

At the moment, the data on professional baseball outside of the Major Leagues, as seen, for example, on baseball-reference.com, are a mashup which combines a number of secondary datasets produced over the years by various researchers. Most of these datasets focused on information of interest to the researcher, and therefore their coverage was targeted to particular leagues, people, and/or types of information. By combining them, we have been able to produce a first cut at an encyclopedic dataset which has broad coverage of much of baseball history.

The resulting mashup is useful, but very incomplete. The limited scope of each individual secondary dataset means there are many, many gaps in coverage. Furthermore, because the datasets were produced to solve a particular need for the researcher, and not necessarily to be part of a systematic encyclopedia, information on the provenance of specific data values varies significantly across the datasets. Although our experience is the information in our dataset is generally reliable when it is present, it is usually not possible to identify a primary source for most of it. And, when a data value is null (or absent) in our data, it is difficult to say whether this is because the information was not ever recorded, or simply was not recorded in one of the secondary datasets.

We are now building a new, comprehensive dataset covering the history of professional baseball. We find ourselves in a situation that is quite similar to that faced by ICI when they started to produce the dataset that became the original Macmillan Baseball Encyclopedia, and we are following a strategy which in many ways is the same as the one they adopted. We are going back to the most primary sources we can find, and building a new dataset “from scratch,” where each data value is traceable back to a source. Of course, we are benefitting greatly from the massive progress that has been made in computer hardware, and in software for organising, transforming, and merging large datasets, in the half a century since the Macmillan appeared, but in many ways our research principles and strategy would be instantly recognisable to anyone who worked on the Big Mac.

One important difference between the late 1960s and today is that dissemination of data is so much faster. Although the Macmillan Encycloepdia was famously the first book to be typeset by computer, it took hours to do the typesetting, and of course the only way to disseminate the information was by printing physical books. Today, we are able to update websites in minutes, or seconds, and people can interact with the data in a variety of ways. Therefore, unlike ICI, we do not need to wait to complete our work to publish it; rather we can disseminate new information on a rolling basis.

To accomplish this in an efficient fashion, we have developed a structured approach to building our new dataset. We organise our work into well-defined blocks, and when each block is ready, it is merged into the dataset and disseminated. We do this in a way that maintains continuity for all of the identifiers associated with people, leagues, teams, and so on, so this updating process should largely be transparent to users: over time, the quality of the data simply gets better. The way to think about this process is that we are not “changing” or “editing” the data values from the legacy mashup; we are replacing them block by block, with a new dataset that is built directly from sources, and that we are able to improve iteratively going forward as we incorporate new sources of information.

Our strategy

To produce a dataset of this size efficiently requires having a good way of organising work into coherent blocks we can use to replace chunks of existing information. We provide below a high-level outline of our strategy.

League statistics

We work with a given league-season as the unit of statistical compilation. Most of the historical statistics in the original mashup came from an impressive database compiled by Ed Washuta. Ed’s work was comprehensive in breadth, going back to 1901, but limited in the statistical columns it covered.

We are currently working backwards in time (similar to Retrosheet), replacing the statistical coverage from Ed’s dataset. For each league-season, we identify the most complete source(s) for league-level performance. Often, the official averages published by the league (and frequently appearing in, e.g., a Guide) is the best existing source. In some cases, we also have work done by researchers which provides averages for leagues which did not publish them, or which improves on what was published by adding missing players (including “less-thans”), additional statistical detail, or correcting errors in statistical tabulation or person names.

As a further stage, we have developed a methodology for efficiently capturing from newspapers boxscores, game logs, and identifying information on individual players. We use this methodology to fill in additional information for a league, especially for leagues where official averages were not published, or were sketchy or otherwise clearly unreliable. We produce updated versions of a league-season at the league-season level. This means that, until we have worked through a league-season completely, it is possible, and in many situations probable, that player lists or statistical totals for that league-season are incomplete.

Person identification

Players, managers, and other personnel are usually incompletely identified in published averages and in newspaper accounts. To link up each person’s baseball career, and to fill in additional identifying and biographical details, we have developed a structured approach for establishing identification based on principles from records linkage. The output of our work forms the Chadwick Register, the most comprehensive collection of identifications of baseball people.

An entry in the Register is a collection (or “cluster”) of references to records and mentions of people in source data, linked together by matching on information in the respective sources. As we are able to link more and more source records into clusters, we increase the amount of information we can present about people, as well as our confidence in the likely accuracy of that information.

There is an enormous amount of historical material available, including published material and official documents - far more than there’s the capacity to capture and link. We therefore have developed a strategy which maximises the rate at which we can incorporate new information efficiently, largely by working systematically through certain types of source material. These include:

  • The Sporting News contract cards, available on the LA84 Foundation’s website, contain useful data on players and other personnel active from about 1920 to 1980, with some limited or sketchy information from circa 1885 to 1919. The content of the cards varies widely. Some players who played just a few minor league games (or were signed but never played at all) have very complete information, while many with significant minor league careers have little information beyond their name and a list of transactions. We are finding that a majority of the cards do have some information which is useful for improving our data quality.
  • Thanks to a partnership with Baseball Reference, we are indexing and abstracting information from the questionnaires which have been scanned and are available on ancestry.com. These documents are quite useful for filling in basic biographical details, and especially schools attended (which are rarely recorded systematically).
  • Thanks to Pete Palmer, we have an improved version of the person biographical table from the Howe Sports Bureau, with coverage from the early 1980s to the early 2000s.
  • Stalwart 19th century baseball researcher Reed Howard has contributed an extensive listing of the careers and biographical details for players and other personnel from the 1860s through to the early 20th century, with source citations.

The person details we can glean from the above sources are often still incomplete. One of the achievements of the Macmillan Encyclopedia was not just getting historical statistics organised and checked, but also providing biographical details for players on a much more systematic basis. The core of that information was information that had been collected over the years by Lee Allen, who was the historian at the Hall of Fame in the 1960s and who had a special interest in knowing details about Major League players’ lives outside of baseball. To collect this type of information for our Register, we have convened the Lee Allen group, an informal network of researchers who collect data on baseball people using sources from outside baseball, including obituaries, vital records, and the like. This group is responsible for reporting the passings of former players and personnel in near-real time. In addition, once we have linked contract cards, profiles, or questionnaires to Register people, but information on them is still missing, they have a go at seeing if they can find information from other sources to fill out demographics in the Register.

As we progress with the records linkage process, as part of the Register we will make available information about the records in each person’s identity cluster. This includes references to person IDs in other systems, such as MLBAM and Wikidata. It also includes references to other source materials which are not necessarily one-to-one with people; for example, we provide references to TSN contract cards and findagrave.com memorials.

Reconstructing correct identifications of people from historical records can be tricky, given the incompletness, ambiguity, and sometimes downright inaccuracy of records. The documentation of the Register outlines our technical approach, including how Chadwick person cluster identifiers evolve to reflect our constantly-improving understanding. Our objective is to optimise the rate we are able to improve the overall quality of the composite data we provide. Therefore, it’s not per se our objective, in the short run, for any of those cross-references to be comprehensive. For example, we have linked only around 15% of the TSN contract cards to date. It will be quite some time before this approaches full linkage, as we prioritise matching cards which help the most in improving the biographical and career data that people see.


Our historical data will always be incomplete and a work in progress. We estimate that at least 250,000 people have played at least one game in one of the over 5,000 seasons played in a professional league. The incompleteness of sources - not to mention the errors and ambiguities within and across sources - will always be limitations, as well as the sheer amount of work it takes to organise the information scattered across those sources. It should be noted that almost all of the work in producing historical data prior to 1984 is being done by volunteer researchers; without their generosity with their time and talents, very little of the work described above would happen.

The description above gives an idea of what a data user can expect of our historical data, what our roadmap is for developing it, and what reasonable timelines are for how that development will happen.

We are always looking for new researchers to support the projects we are working on. We know our approach of working systematically through a given source does not appeal to everyone, but we find it rewarding to see the quality of our data improve day by day. If you think you might be interested in participating, please do get in touch. Our system is designed such that we can efficiently incorporate contributions of both small and large amounts of work - so it is possible to make useful contributions with just a few hours of work here and there.