Priors for Unrated Players

When Ratings Central processes an event, we first assign a prior mean and standard deviation to each unrated player in the event. (The adjective “prior” means that it is the player's mean and standard deviation prior to the start of the event.) If you specified a prior mean and standard deviation for the player, then we use those values. If you didn't specify a prior mean and standard deviation for the player, then we use the prior mean and standard deviation that you specified for the event.

The following sections give detailed advice on specifying the individual player prior means and standard deviations and the event prior mean and standard deviation. If you have any questions or aren't sure what to do, please for assistance.

Please get unrated players at your events as many matches with rated players of a similar playing level as you can. This will make the resulting ratings for these players more accurate.

Note that the Ratings Central rating scale (i.e., what playing strength a number corresponds to) is similar to the USATT rating scale. This scale is not the same as the ITTF rating scale or the CTTA rating scale.

Player Priors

Please specify the prior mean and standard deviation for as many unrated players as you can. Generally, you should specify the prior mean and standard deviation for an unrated player only if you have additional information about that player. However, the “additional information” can be any information other than the player's match results in the event (we see those). For example, you might know the player from before the event or you might watch the player play his matches or you might look at how many points the player scored in his matches.

If a player is different from the general population of unrated players at the event, e.g., much better or much worse (perhaps because he is very young), then it would be an especially good idea to specify the player's prior mean and standard deviation.

If many or most of the players in your event are unrated, then it is especially helpful to the rating system if you can specify the prior mean and standard deviation for as many of the unrated players as you can.

The prior standard deviation for a player measures how sure you are that you know that player's playing strength. You should be willing to bet at 1:2 odds that the player's playing strength is within one standard deviation of the mean, and you should be willing to bet at 2:1 odds that the player's playing strength is more than one standard deviation from the mean. (Odds of 1:2 mean that you win $1 if you win the bet, but you lose $2 if you lose the bet. Odds of 2:1 mean that you win $2 if you win, but you lose $1 if you lose.)

For example, suppose you assign a prior mean of 1200 and a prior standard deviation of 100 to a player. Then you should be willing to bet at 1:2 odds that the player is really between 1100 and 1300, and you should be willing to bet at 2:1 odds that the player is really less than 1100 or more than 1300. Equivalently, you should believe that there is a 2/3 chance that the player is really between 1100 and 1300 and a 1/3 chance that the player is really less than 1100 or more than 1300.

Here are some very rough guidelines: If you know an unrated player extremely well (e.g., he plays at your club every week), then you might use a prior standard deviation of 50–75. If you only know a player moderately well (e.g., he's come to your club a few times and played several matches with players of a similar level), then you might use a prior standard deviation of 100–125. If you know very little about a player (e.g., you had the player hit with a rated player of a similar level for five minutes), then you might use a prior standard deviation of 150–200.

If a player has a USATT rating, then that is a source of information. However, USATT ratings vary widely in their accuracy, so it is best to check that a player's USATT rating seems reasonable using your own knowledge of the player. USATT ratings do not come with standard deviations, so you will need to make up a prior standard deviation for such a player. If all you know about a player is their USATT rating, then we suggest using a prior standard deviation of 100 for players rated at least 1200 and a prior standard deviation of 200 for players rated less than 1200.

Event Prior

You must always set the event prior mean and standard deviation.

While the prior mean and standard deviation for a player measure what you know of the player's playing strength, it is best to interpret the event prior mean and standard deviation as describing the range of unrated players at your event. For example, if you think the unrated players range from 800 to 1400, then you would use the average of these two values (i.e., 1100) as the mean and the difference of these two values divided by 4 (i.e., 150) as the standard deviation.

More precisely, about 2/3 of the unrated players should be within one standard deviation of the mean (and about 1/3 should be more than one standard deviation from the mean), 95% should be within two standard deviations, and 99.7% should be within three standard deviations. So, for the example in the previous paragraph of a mean of 1100 and a standard deviation of 150, you should think that

• 2/3 of the players are between 950 and 1250,
• 95% of the players are between 800 and 1400,
• 99.7% of the players are between 650 and 1550.

Note that when estimating the event prior standard deviation from the range of players you expect at your event, you should interpret the range as being plus or minus two standard deviations, not three.

If you specify the prior mean and standard deviation for any individual unrated players, then the event prior mean and standard deviation should only describe the population of unrated players for whom you haven't specified individual prior means and standard deviations. If you set the prior mean and standard deviation individually for every unrated player at your event, then Ratings Central won't actually use the event prior mean and standard deviation, but you still have to set them.