Standard Deviation (SD)

Also known as: std dev, SD, sigma

The square root of variance; the single number that quantifies how widely your results spread, usually in bb/100.

Standard deviation (\(\sigma\)) is the square root of variance and the standard unit for measuring spread. In cash it's quoted in bb/100; in tournaments, in ROI points or buy-ins per event.

The defining property is how it scales with sample size. Per-hand SD is \(\sigma\); over \(N\) hands the SD of total profit grows as \(\sigma\sqrt{N}\). Your expected profit grows as \(wN\) (linear), so the ratio of edge to noise improves only as \(\sqrt{N}\). This single asymmetry is the entire reason poker is a long-run game.

Benchmark figures worth memorising:

• 6-max NLHE cash: \(\sigma \approx 80\text{–}100\) bb/100. Looser, more aggressive styles push toward 100+; tight styles lower.
• Full-ring cash: ~60–75 bb/100.
• PLO: higher, often 100–130+ bb/100 — fund it with more buy-ins.
• MTTs: per-tournament SD can be several times the mean ROI; this is why MTT rolls run 50–100+ buy-ins.

SD feeds directly into risk of ruin (\(\sigma^2\) in the denominator of the exponent) and into how big a sample you need to trust a win rate. A confidence interval on your win rate after \(N\) hands is roughly \(w \pm 1.96\,\sigma/\sqrt{N}\) per 100 hands.