Editing Elo rating system (section)

====Performance rating====
{{See also|Performance rating (chess)}} 
{| class="wikitable" style="width:100px; float:right; text-align:center; margin-top:0; margin-left:10px;"
|-
! style="width:50%;"|<math>p</math>
! style="width:50%;"|<math>d_p</math>
|- 
| 1.00 || +800
|- 
| 0.99 || +677
|-
| 0.9 || +366
|-
| 0.8 || +240
|-
| 0.7 || +149
|-
| 0.6 || +72
|-
| 0.5 || 0
|-
| 0.4 || −72
|-
| 0.3 || −149
|-
| 0.2 || −240
|-
| 0.1 || −366
|-
| 0.01 || −677
|-
| 0.00 || −800
|}
Performance rating or special rating is a hypothetical rating that would result from the games of a single event only. Some chess organizations{{r|uschess2020|p=p. 8}} use the "algorithm of 400" to calculate performance rating. According to this algorithm, performance rating for an event is calculated in the following way:
# For each win, add your opponent's rating plus 400, 
# For each loss, add your opponent's rating minus 400, 
# And divide this sum by the number of played games.

Example: 2 wins (opponents {{mvar|w}} & {{mvar|x}}), 2 losses (opponents {{mvar|y}} & {{mvar|z}})<br />
<!--Please see Talk page for the formatting changes I made here. Thanks!-->
:<math>
\begin{align}
& \frac{w+400+x+400+y-400+z-400}{4} \\[6pt]
& \frac{w+x+y+z+400(2)-400(2)}{4}
\end{align}
</math>
This can be expressed by the following formula:
:<math> \text{performance rating} = \frac{\text{total of opponents' ratings } + 400 \times (\text{wins} - \text{losses})}{\text{games}}</math>
Example: If you beat a player with an Elo rating of 1000,
:<math>
\text{performance rating} = \frac{1000 + 400 \times (1)}{1} = 1400</math>
If you beat two players with Elo ratings of 1000,
:<math> \text{performance rating} = \frac{2000 + 400 \times (2)}{2} = 1400</math>
If you draw,
:<math> \text{performance rating} = \frac{1000 + 400 \times (0)}{1} = 1000</math>

This is a simplification, but it offers an easy way to get an estimate of PR (performance rating).

[[FIDE]], however, calculates performance rating by means of the formula<math display="block">\text{performance rating} = \text{average of opponents' ratings} + d_p,</math>where "rating difference" <math>d_p</math> is based on a player's tournament percentage score <math>p</math>, which is then used as the key in a lookup table where <math>p</math> is simply the number of points scored divided by the number of games played. Note that, in case of a perfect or no score <math>d_p</math> is 800.
{{Clear}}