Is WTN necessary?

Among amateur tennis players, including high school and college players, I think it is fair to say that UTR (Universal Tennis Rating) is the number that we look at to measure the skill level. UTR rates tennis players by a number between 1 and 16 something, with a higher number indicating higher skill level. According to the Wikipedia page, UTR was developed by a private company in 2008 to evaluate junior players.

In 2020, perhaps to compete against UTR, the International Tennis Federation (ITF) introduced WTN (World Tennis Number). WTN also rates tennis players by a number between 1 and 40, but with a lower number indicating higher skill level.

My impression with looking at both numbers is that UTR is more intuitive (because larger number means higher skill) and also more accurate. The reason I think UTR is more accurate is only based on anecdotal evidence. For instance, I have seen some adult and junior players that I have a good idea about their skills rated in the opposite way based on UTR and WTN, and my impression of their skills was closer to UTR. Another reason I have a strong prior that UTR is more accurate is that UTR uses more data. Because WTN is backed by ITF, it only counts official matches sanctioned by the tennis federations of each country, for instance USTA in United States. But because USTA tournaments are divided by age and gender, for instance a 13 year old girl never plays against a 17 year old boy, which makes it impossible to compare their skill level. In contrast, in addition to counting all official matches, UTR also organizes co-ed UTR tournaments and flex leagues, so there are more opportunities to compare the skill level.

My daughter is a junior player and she has been analyzing past opponents by keeping track of their UTR, play style, strengthes and weaknesses, etc., in a spreadsheet. The picture below shows the scatter plot of WTN against UTR. The markers in blue, read, and yellow correspond to the players in her group lesson, tournament opponents living around San Diego, and those living around Los Angeles. We can make a few observations. First, all markers show downward-sloping patterns, so a high UTR corresponds to a low WTN, as it should be. Second, the San Diego and Los Angeles markers show the similar straight-line pattern, so the relation between UTR and WTN seems to be the same in San Diego and Los Angeles. This is natural because my daughter often travels to Orange County to participate in tournaments, and there are also many players from Orange County that plays in San Diego tournaments. Third, which is the most interesting point, is that Group markers are off from the San Diego/Los Angeles markers, except three points corresponding to UTR around 6. It turns out that these three points correspond to girls, and the remaining eight points that are off correspond to boys. This is evidence that WTN is less accurate: because boys and girls never play official matches against each other, WTN has no way to compare their skill levels, whereas UTR can sometimes compare due to the presence of co-ed tournaments.

In summary, there is no need to look at WTN.