Just how is this bias calculated? From admissions from the refs? It's fucking retarded.
the link above had the following ;
Methodology and Limitations
For this study, we gathered game data from the five largest leagues in European football (English Premier League, La Liga, Serie A, Bundesliga, and Ligue 1) from the last 10 years using the
FootyStats API. As a result, we acquired match data for 18,227 matches. We filtered out matches for which card data and referee name wasn’t reported, leaving us with 9,960 matches. For each match, we recorded the home and away clubs, the number of yellow and red cards given to both clubs, and the referee in charge of the match. Using this information, we were able to analyse referee behaviour individually, toward each club, and against each club.
No statistical testing was performed, so the claims listed above are based on means alone. As such, this content is exploratory and is presented for informational purposes only. The API pull took place during August 2021.