Position Sizing Based on Confidence Intervals
Trading Intelligence
8 min read
Size positions based on statistical certainty rather than emotion, using confidence intervals from your actual track record.
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8 min read
Size positions based on statistical certainty rather than emotion, using confidence intervals from your actual track record.
Just because you had 10 wins in a row doesn’t mean you should double your risk. Here’s how to size based on statistical certainty — not emotion.
Most traders ask:
“Can I size up now?”
And most do it based on:
But professionals ask:
“Is my edge proven enough to justify larger size?”
This post introduces confidence intervals — a statistical way to decide when it’s safe to scale up, and how much your data can be trusted.
A confidence interval (CI) tells you:
“Given my sample of results, what’s the range of possible true values for my win rate, EV, or Sharpe ratio?”
It’s based on:
Let’s say you’ve had 20 trades with a +0.5R average return.
With a small sample, your confidence interval is wide — meaning the true EV could be much lower (or higher).
You shouldn’t size up until your edge is statistically stable.
You win 12 out of 20 trades → 60% win rate
95% Confidence Interval for win rate (binomial CI):
≈ 38% to 79%
That means: You're 95% confident that your true win rate lies somewhere in that range. That’s… not very reliable.
Now with 100 trades and 60 wins: CI shrinks to ≈ 50% to 69% → Far more stable → safer to scale risk slightly
Let’s say:
The 95% CI for the mean EV is:
EV ± 1.96 × (σ / √n)
= 0.6 ± 1.96 × (1.4 / √25)
= 0.6 ± 1.96 × 0.28
= 0.6 ± 0.55
→ EV Range = [0.05R, 1.15R]
That’s a huge range. Would you want to size up based on that?
Now try 100 trades:
EV ± 1.96 × (1.4 / √100)
= 0.6 ± 0.27 → [0.33R, 0.87R]
Now you’re more confident in your edge — and can justify incremental risk increases.
| Trade Count (Sample Size) | Recommended Max Risk per Trade |
|---|---|
| 0–30 trades | 0.25%–0.5% |
| 30–75 trades | 0.5%–0.75% |
| 75–150 trades | 1.0% |
| 150+ trades, stable stats | 1.25%–1.5% (advanced only) |
Scaling tip: Only increase size if:
You wouldn't bet millions on 10 trades. Don't risk a large % of your capital based on a small data set.
Use confidence intervals to protect yourself from overconfidence.
Real edge isn’t just about being profitable — it’s about knowing how stable that profitability is.
Confidence intervals give you:
Let your system earn the right to scale.