{
  "study": "backtest-sampling-error",
  "title": "How Wrong Is a 30-Trade Backtest? Sampling Error Measured",
  "description": "Your backtest says 55%. How far from the truth could that be? We measured it: at 30 trades the answer is off by more than 10 percentage points 28.4% of the time, and 90% of results land in a band 30 points wide. Here is the error bar for every sample size.",
  "source": "https://fxbacktest.app/research/backtest-sampling-error/",
  "publisher": "FxBacktest",
  "license": "https://creativecommons.org/licenses/by/4.0/",
  "attribution": "FxBacktest, https://fxbacktest.app/research/backtest-sampling-error/",
  "datePublished": "2026-08-24",
  "dateModified": "2026-09-08",
  "generated": "2026-09-18",
  "tables": [
    {
      "id": "what-each-sample-size-actually-buys",
      "caption": "What each sample size actually buys you",
      "captionSource": "heading",
      "columns": [
        "Trades in your backtest",
        "90% of backtests show",
        "How wide that is",
        "Give or take",
        "Typical miss",
        "Badly wrong (>10 pts off)"
      ],
      "rowCount": 8,
      "rows": [
        {
          "Trades in your backtest": 10,
          "90% of backtests show": "20.0% – 80.0%",
          "How wide that is": "60.0 pts",
          "Give or take": "±30.0 pts",
          "Typical miss": "12.33 pts",
          "Badly wrong (>10 pts off)": 55.3
        },
        {
          "Trades in your backtest": 20,
          "90% of backtests show": "30.0% – 70.0%",
          "How wide that is": "40.0 pts",
          "Give or take": "±20.0 pts",
          "Typical miss": "8.89 pts",
          "Badly wrong (>10 pts off)": 38.6
        },
        {
          "Trades in your backtest": 30,
          "90% of backtests show": "33.3% – 63.3%",
          "How wide that is": "30.0 pts",
          "Give or take": "±15.0 pts",
          "Typical miss": "7.30 pts",
          "Badly wrong (>10 pts off)": 28.4
        },
        {
          "Trades in your backtest": 50,
          "90% of backtests show": "38.0% – 62.0%",
          "How wide that is": "24.0 pts",
          "Give or take": "±12.0 pts",
          "Typical miss": "5.65 pts",
          "Badly wrong (>10 pts off)": 15.9
        },
        {
          "Trades in your backtest": 100,
          "90% of backtests show": "42.0% – 58.0%",
          "How wide that is": "16.0 pts",
          "Give or take": "±8.0 pts",
          "Typical miss": "3.96 pts",
          "Badly wrong (>10 pts off)": 4.5
        },
        {
          "Trades in your backtest": 200,
          "90% of backtests show": "44.0% – 55.5%",
          "How wide that is": "11.5 pts",
          "Give or take": "±5.8 pts",
          "Typical miss": "2.79 pts",
          "Badly wrong (>10 pts off)": 0.4
        },
        {
          "Trades in your backtest": 500,
          "90% of backtests show": "46.2% – 53.6%",
          "How wide that is": "7.4 pts",
          "Give or take": "±3.7 pts",
          "Typical miss": "1.77 pts",
          "Badly wrong (>10 pts off)": 0
        },
        {
          "Trades in your backtest": 1000,
          "90% of backtests show": "47.3% – 52.5%",
          "How wide that is": "5.2 pts",
          "Give or take": "±2.6 pts",
          "Typical miss": "1.26 pts",
          "Badly wrong (>10 pts off)": 0
        }
      ]
    }
  ]
}