Answers.Body: <p>I believe that both sites use the same engine to perform game analysis, stockfish. If they use different engines (possible), then the answer is trivial: Over the span of 39 moves the engines opinion differs enough to result in a 13% accuracy difference.<br />
  Assuming the engines are the same, they may just be using different depths. The same engine at different depth is likely to produce a different evaluation.<br />
  Assuming the engines are the same and using the same depth, engine analysis <a href="https://chess.stackexchange.com/questions/19482/randomness-in-engine-play">isn't deterministic</a>. This means that it's possible to get a different result if you run the engine twice on one position to the same depth. Secondly, the depth of these full-game accuracy scores is fairly shallow. You can see this in action when you pull up a position in lichess that was assigned an evaluation via game review. If you let stockfish run past depth ~20, you'll see a change in evaluation. The shallowness of the search would likely exacerbate the randomness of the evaluation because each engine has a greater chance of a subtle misevaluation at shallow depths.</p>
  
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2024-10-25T00:16:20.770Z
Answers.Text: This is a good answer and addresses part of the issue. The sites also structurally calculate accuracy in similar but non-identical ways.
Answers.UserDisplayName: SecretAgentMan
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2024-10-28T17:29:06.633Z
Answers.Text: @SecretAgentMan I assume you're talking about how the sites converts centipawn loss to accuracy? Honestly, I'm not quite sure how each site performs that calculation. I'm sure it matters, probably even more-so than the reasons I gave!
Answers.UserDisplayName: Awalrod
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2024-10-24T15:44:09.697Z
Answers.LastActivityDate: 2024-10-24T15:44:09.697Z
Answers.UserDisplayName: Awalrod
Answers.Body: <p>I have been wondering about that, too. Even seen chess.com evaluate the very same move differently on web vs. mobile app.</p>
  <p>Assuming that these calculations aren't standardized or regulated, because they are only supportive, but not relevant for the actual game.<a href="https://i.sstatic.net/oTfE27eA.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/oTfE27eA.jpg" alt="chess.com web vs. mobile eval for the same move" /></a></p>
  <p>(full insta post <a href="https://www.instagram.com/p/CnFFiGFjYF-/?utm_source=ig_web_copy_link&amp;igsh=MzRlODBiNWFlZA==" rel="nofollow noreferrer">https://www.instagram.com/p/CnFFiGFjYF-/?utm_source=ig_web_copy_link&amp;igsh=MzRlODBiNWFlZA==</a>).</p>
  
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2024-11-06T10:21:14.117Z
Answers.LastActivityDate: 2024-11-06T10:21:14.117Z
Answers.UserDisplayName: Chess Maps
Body: <p>Take this game for example.</p>
  <pre><code>[FEN &quot;&quot;]
   1. e4 c5 2. Nf3 e6 3. d4 cxd4 4. Nxd4 a6 5. Bd3 Nf6 6. Bg5 Bc5 7. Nf3 h6 8. Bh4 b5 9. a3 Bb7 10. Nc3 d6 11. Qd2 Nbd7 12. Be2 Rc8 13. Qd3 Qc7 14. Bg3 e5 15. Nd2 Bd4 16. Ndb1 Nb6 17. Ra2 O-O 18. O-O Rfd8 19. Rc1 Qc4 20. Qxc4 Nxc4 21. Bxc4 Rxc4 22. Re1 Nxe4 23. Nxe4 Bxe4 24. Rxe4 Bxf2+ 25. Kxf2 Rxe4 26. Nc3 Rd4 27. Ke3 f5 28. Be1 f4+ 29. Ke2 d5 30. Bf2 Rc4 31. Kd3 e4+ 32. Ke2 e3 33. Bh4 g5 34. Be1 h5 35. Ra1 g4 36. Rd1 d4 37. Na2 Rxc2+ 38. Kf1 f3 39. gxf3 e2+ 0-1
  </code></pre>
  <p>Here are the two accuracies that chess.com and lichess gave me (as the Black pieces):</p>
  <ul>
  <li>Chess.com Game Review: <strong>72.1%</strong></li>
  <li>Lichess Game Analysis: <strong>85%</strong></li>
  </ul>
  <p>So Lichess gives me (much) higher accuracies that chess.com. Why is this? Do they use different evaluation methods or something?</p>
  
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2024-10-24T11:30:55.187Z
Comments.Text: "Do they use different evaluation methods" - Yes, that's the only way you can different answers. Quite *how* they differ needs research if you want to find out. It could be the same algorithm with different underlying engines, or different algorithms, or probably any number of other variables - I have no idea.
Comments.UserDisplayName: Ian Bush
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2024-10-24T13:18:58.003Z
Comments.Text: Related (not a dupe): [Is there a way to get blunders, mistakes and inaccuracies using Stockfish?](https://chess.stackexchange.com/q/41396/26335)  This has some relevant information and links regarding how the site's calculate accuracy. Again, not a dupe.
Comments.UserDisplayName: SecretAgentMan
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2024-11-06T11:00:26.303Z
Comments.Text: Is the definition of "% accuracy" actually standardised across platforms?
Comments.UserDisplayName: Steve Bennett
Tags: <engines><lichess><chess.com>
Title: Why do Lichess and Chess.com give different accuracies for the same game?
ThreadText: # Why do Lichess and Chess.com give different accuracies for the same game?
  
  (Asked by: username_0 on 2024-10-24)
  
  Take this game for example.
  
  ```
  [FEN ""]
   1. e4 c5 2. Nf3 e6 3. d4 cxd4 4. Nxd4 a6 5. Bd3 Nf6 6. Bg5 Bc5 7. Nf3 h6 8. Bh4 b5 9. a3 Bb7 10. Nc3 d6 11. Qd2 Nbd7 12. Be2 Rc8 13. Qd3 Qc7 14. Bg3 e5 15. Nd2 Bd4 16. Ndb1 Nb6 17. Ra2 O-O 18. O-O Rfd8 19. Rc1 Qc4 20. Qxc4 Nxc4 21. Bxc4 Rxc4 22. Re1 Nxe4 23. Nxe4 Bxe4 24. Rxe4 Bxf2+ 25. Kxf2 Rxe4 26. Nc3 Rd4 27. Ke3 f5 28. Be1 f4+ 29. Ke2 d5 30. Bf2 Rc4 31. Kd3 e4+ 32. Ke2 e3 33. Bh4 g5 34. Be1 h5 35. Ra1 g4 36. Rd1 d4 37. Na2 Rxc2+ 38. Kf1 f3 39. gxf3 e2+ 0-1
  
  ```
  
  Here are the two accuracies that username_5.com and lichess gave me (as the Black pieces):
  
  * username_5.com Game Review: **72.1%**
  * Lichess Game Analysis: **85%**
  
  So Lichess gives me (much) higher accuracies that username_5.com. Why is this? Do they use different evaluation methods or something?
  
  **Tags:** engines, lichess, chess.com
  
  > **Comments:**
  > **username_1**: "Do they use different evaluation methods" - Yes, that's the only way you can different answers. Quite *how* they differ needs research if you want to find out. It could be the same algorithm with different underlying engines, or different algorithms, or probably any number of other variables - I have no idea.  
  > **username_2**: Related (not a dupe): [Is there a way to get blunders, mistakes and inaccuracies using Stockfish?](https://username_5.stackexchange.com/q/41396/26335)  This has some relevant information and links regarding how the site's calculate accuracy. Again, not a dupe.  
  > **username_3**: Is the definition of "% accuracy" actually standardised across platforms?  
  
  ---
  
  ## 2 answers
  
  ### Answer by username_4 on 2024-10-24. Score: 4
  
  I believe that both sites use the same engine to perform game analysis, stockfish. If they use different engines (possible), then the answer is trivial: Over the span of 39 moves the engines opinion differs enough to result in a 13% accuracy difference.  
  Assuming the engines are the same, they may just be using different depths. The same engine at different depth is likely to produce a different evaluation.  
  Assuming the engines are the same and using the same depth, engine analysis [isn't deterministic](https://username_5.stackexchange.com/questions/19482/randomness-in-engine-play). This means that it's possible to get a different result if you run the engine twice on one position to the same depth. Secondly, the depth of these full-game accuracy scores is fairly shallow. You can see this in action when you pull up a position in lichess that was assigned an evaluation via game review. If you let stockfish run past depth ~20, you'll see a change in evaluation. The shallowness of the search would likely exacerbate the randomness of the evaluation because each engine has a greater chance of a subtle misevaluation at shallow depths.
  
  > **Comments:**
  > **username_2**: This is a good answer and addresses part of the issue. The sites also structurally calculate accuracy in similar but non-identical ways.  
  > **username_4**: @username_2 I assume you're talking about how the sites converts centipawn loss to accuracy? Honestly, I'm not quite sure how each site performs that calculation. I'm sure it matters, probably even more-so than the reasons I gave!  
  
  ---
  
  ### Answer by username_5 on 2024-11-06. Score: 1
  
  I have been wondering about that, too. Even seen username_5.com evaluate the very same move differently on web vs. mobile app.
  
  Assuming that these calculations aren't standardized or regulated, because they are only supportive, but not relevant for the actual game.[![username_5.com web vs. mobile eval for the same move](https://i.sstatic.net/oTfE27eA.jpg)](https://i.sstatic.net/oTfE27eA.jpg)
  
  (full insta post <https://www.instagram.com/p/CnFFiGFjYF-/?utm_source=ig_web_copy_link&igsh=MzRlODBiNWFlZA==>).
  
  
  ---
  
  