This is a description of Swiss method under the US Chess/General Rule Set. The FIDE Dutch System has a somewhat different method for choosing floats.

 

When pairing rounds two and later, it is very rare to have an even number of players in each score group. When there is an odd number in a group, someone must either be raised into a higher group ("upfloated") or dropped into a lower one ("downfloated"). So long as there are quite a few players in each score group, it is almost always possible to simply work down from the high score group to the low one, dropping a single player into the next pack down whenever there is an odd number left in a group.

 

There are two preference items (in the Pairing Rules tab of the Preference Dialog) which help determine which player is dropped in these situations. There is also a Keep in Score Group switch in the Tournament Info tab of the Add/Edit a Player dialog that can be used to keep certain players in their score group, and an Avoid Drops setting in the Pair a Round dialog box which can be used to limit drops. All of these options are described below.

 

Preference Settings

 

The first is Avoid Dropping Unrated. This follows US Chess Rule 29D1, that the lowest ranked player, but not an unrated, is to be the player dropped. If you leave this unchecked, unrateds will be dropped first. Obviously, if there are no rated players in a group, an unrated has to be dropped. However, even if Avoid Dropping Unrated is checked (which we recommend), WinTD may choose to drop an unrated if the alternative is seriously distorted pairings (meaning rating shifts above the "high" limit).

 

Harkness (Drop Middle) is a older method of handling odd score groups which isn't documented in the rule book, but which has certain nice features. Under this option, the player dropped will usually be the middle player; that is, if there are 25 in the score group, the 13th ranked player would be dropped. The logic behind this is that the pairings of all players stay pretty similar if one extra player is added to a score group. With Harkness, the dropped player goes from playing the top of his own score group to the top of the next (probably a rather modest rating difference), while with conventional pairings the dropped player changes from playing the middle of his own score group to the top of the next. Harkness also gives the high player in the next lower score group a tougher pairing.

 

Conventionally, the player raised from the next score group to play the dropped player is the highest rated player in that group. However, the choice of which player to drop and which to raise can be changed to correct colors and other pairing problems. In deciding whether to change the player dropped or raised, WinTD evaluates changes by comparing a dropped player's rating with that of the natural dropped player (based upon your preference choices) or the raised player's rating with that of the highest rating in the section. For instance, if the lowest rated player in the section has a rating of 1205, then dropping a 1238 instead is a 33 point change. The rating change on the other side is ignored, since those players are in different score groups (apples and oranges). Similarly, if the highest rated player in a score group is 1620, raising a 1530 instead is 90 points. If we switch both ends (drop the 1238 to play the 1530), add the two differences to get 123.

 

WinTD is much more likely than the typical pairing program to make these types of adjustments because it doesn't pair score group by score group. You may find that you get some questions from the players who aren't used to these types of pairing changes. If need be, you can quote rules 29D1 and 29D2 ("Care must be taken in doing this that...the color consequences are acceptable" and "It is acceptable to pair the player against a somewhat lower rated player to equalize or alternate colors...."). However, we have offered a preference if this type of pairing change isn't to your liking as a TD. This is Avoid Drop/Raise Swap in Color Allocation Rules. This doubles the rating differences in these situations, which makes it highly unlikely that anything more than a trivial change will be made.

 

When score groups get smaller (in a small tournament, or later rounds in a larger one), it might not be possible to get by with a series of single drops. For instance, suppose that there are two players at 3.0, two at 2.5, and a bunch at 2.0, but the 2.5's have played each other. There are two ways to handle this: the 2.5's can be raised to play the 3.0's, or the 2.5's can be dropped to play 2.0's. WinTD will very strongly favor the latter—the higher score groups are kept intact if possible. However, WinTD will choose to raise the 2.5's instead if dropping them forces additional multiple drops out of the 2.0 group. When faced with complicated situations, WinTD tries to minimize the total number of points involved in drops (a 2.5 playing a 2.0 counts as 0.5, or 2.0 playing a 1.0 counts as 1.0), and, for a given number of points in drops, to try to make a large number of small drops (a 2.5 playing a 1.5 is worse than a 2.5 playing a 2.0 combined with a 2.0 playing a 1.5). When the sizes of the drops are equal, the preference is for drops at lower score levels, which are less likely to distort the standings.

 

Keep in Score Group

 

This is a switch in the Tournament Info tab of the Add/Edit a Player dialog box which can be turned on for individual players. If you turn this switch on, WinTD will try to keep the player in his own score group when pairing (the player will still be dropped or raised if the alternative would mean pairing teammates or clubmates, or duplicating an earlier game). For example, this feature could be used to prevent players who are significantly under- or over-performing their rating level from continually being dropped or raised to different score groups.

 

Preventing Large Drops

 

The Avoid Drops of More Than field on the Miscellaneous Tab on the Pair a Round dialog box allows you to restrict the distance (in terms of score differential) across which a player will be dropped. For example, if you enter the value 2.0 in this field, WinTD will break all other pairing rules (color allocation, pairing teammates or even duplicating an earlier pairing) to avoid dropping a player to a score group more than 2 points below his own. You should be very careful about using this feature—just leave this field blank unless you are absolutely sure you want to do otherwise.

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