Analysis - Factors for Dama

In this analysis, we'll look at several aspects of the game and see how they affect whether a player chooses to riichi or dama a tenpai hand.

In the previous post, we saw that, overall, riichi and dama have similar win rates. One factor to this could be that people choose to dama in more dangerous situations, making other players more likely to win.

One easy measure of how close someone is to tenpai is how many times they've called. Let's see how the rates of riichi and dama change based on the number of calls in the game.

Aside from 10 and 11 calls, which both have only one instance each, the riichi rate steadily decreases as the number of calls goes up. Calling riichi against an open tenpai isn't quite as risky as calling it against another riichi, but it's still dangerous.

How about based on how late into the round it is? The later into the round, the more likely people are tenpai, and if it's very late, the player might not want to throw the riichi stick away.

Naturally, with the 17+ ones, you may not have another draw, which would prevent you from calling riichi at all. The riichi rate slowly declines up to the end of second row, then starts declining sharply on the third row.

Next, let's look at the round itself. If it's a later round, they might not want to take the risk of riichi, since there's no time to fix the mistake.

People strongly prefer dama when it's at the end of the game. After all, there's less reason to riichi in South 4 or West if you have enough points, and the riichi stick might even help someone else surpass you.

What about the player's current points? With more points, keeping your lead safe with damas is an option. With less points, getting more by calling riichi is appealing.

Amusingly, the riichi rate goes down until the 50~50k point, then after that it starts to climb again. At that point your lead is so big you can afford a hit, and might want to riichi to cause people to fold or go bankrupt.

One more thing is the player's hand's value. Since I don't have the capability to calculate this myself yet, we'll just look at the won hands. To find the value before riichi, we'll remove the following yaku from the score: Riichi, Double Riichi, Ippatsu, Tsumo, Ura Dora, Haitei, Houtei, Chankan.

There's a preference to dama the expensive hands. Any hand beyond 60 fu will involve a kan, so those are more tempting to riichi for the extra ura dora.

The raw data for this post can be found in this spreadsheet.