Our measure of field strength is interpreted as the number of strokes per round better (+) or worse (-) the average player in the field is than the average player in a regular (non-major and non-WGC) PGA Tour event.
Some details are provided after the table.
To obtain the measure of field strength, we first calculate each player’s adjusted scoring average (adjusted for course difficulty**) in the year 2017. Overall field strength is then defined as the average adjusted scoring average of all players in the field. Top 25% field strength is the average of the adjusted scoring averages belonging to the best 25% of players in the field. The other % breakdowns are defined similarly.
An example for clarity: the average player competing in the WGC-Bridgestone is about 1 stroke better per round than the average player that competes in a (non-major/non-WGC) PGA Tour event. It turns out that the average player in the Travelers Championship is a good reference point for a typical PGA Tour event in terms of field quality.
** The adjusted score is not simply score minus the field average for the day; we also adjust for the quality of the field. We are able to properly adjust for field strength across the various Tours because all of the events in our data are “connected”, in the sense that the set of players in each tournament overlap, at least to some degree. This allows us to compare players on the PGA Tour to players on the Asian Tour by comparing their performances against common opponents on the European Tour, for example. (In practice, this is accomplished through a 2-way fixed effect regression, which we talk about here and here). Finally, we normalize all adjusted scores so that they are interpreted as the number of strokes-gained over the average player in a regular PGA Tour event (i.e. positive numbers are good, negative are bad).