{
  "metadata": {
    "analysisVersion": 1,
    "sourceRevision": "4c1ff5e3aef1816ae04af63218015066e186c147",
    "sourceUrl": "https://github.com/fivethirtyeight/data/tree/4c1ff5e3aef1816ae04af63218015066e186c147/births",
    "sourceRetrievedAt": "2026-09-15T15:12:26.504696+00:00",
    "baseline": {
      "population": "Historical U.S. birth records",
      "startDate": "1994-01-01",
      "endDate": "2014-12-31",
      "sources": [
        {
          "name": "CDC/NCHS",
          "startYear": 1994,
          "endYear": 1999
        },
        {
          "name": "SSA",
          "startYear": 2000,
          "endYear": 2014
        }
      ],
      "totalBirths": 85712738,
      "observedDays": 7670,
      "monthDayCount": 366
    }
  },
  "septemberNine": {
    "date": "09-09",
    "births": 259228,
    "occurrences": 21,
    "birthsPerOccurrence": 12344.190476190477,
    "rank": 1,
    "share": 0.003024381276911257,
    "oneIn": 330.64614162050395,
    "nextHighest": {
      "date": "09-19",
      "label": "September 19",
      "births": 257981,
      "rank": 2
    },
    "topFive": [
      {
        "date": "09-09",
        "label": "September 9",
        "births": 259228,
        "occurrences": 21,
        "birthsPerOccurrence": 12344.190476190477,
        "rank": 1
      },
      {
        "date": "09-19",
        "label": "September 19",
        "births": 257981,
        "occurrences": 21,
        "birthsPerOccurrence": 12284.809523809523,
        "rank": 2
      },
      {
        "date": "09-12",
        "label": "September 12",
        "births": 257913,
        "occurrences": 21,
        "birthsPerOccurrence": 12281.57142857143,
        "rank": 3
      },
      {
        "date": "09-17",
        "label": "September 17",
        "births": 256231,
        "occurrences": 21,
        "birthsPerOccurrence": 12201.47619047619,
        "rank": 4
      },
      {
        "date": "09-10",
        "label": "September 10",
        "births": 255991,
        "occurrences": 21,
        "birthsPerOccurrence": 12190.047619047618,
        "rank": 5
      }
    ],
    "leadBirths": 1247,
    "leadPercentOfNext": 0.48336893027005867,
    "topTenSeptemberCount": 9,
    "topTwentySeptemberCount": 13,
    "septemberMonthShare": 0.08680925581912924,
    "septemberMonthBirths": 7440659,
    "septemberMonthBirthsPerDay": 11810.569841269842
  },
  "birthdayParadox": {
    "method": {
      "question": "At least one pair in the group shares a month/day birthday.",
      "classic": "365 equally likely month/day categories; February 29 excluded.",
      "historical": "366 month/day categories weighted by their observed shares of CDC/NCHS 1994–1999 and SSA 2000–2014 recorded U.S. births; February 29 retained at its observed share.",
      "assumption": "Independent sampling with replacement from the selected distribution; this is a model, not a measured probability for actual social groups or the current U.S. population.",
      "calculation": "Collision probability = 1 − n! times the nth elementary symmetric sum of category shares. The scaled recurrence updates q[k] += k × p × q[k−1] in descending k for each category. The classic formula multiplies (365−i)/365 for i = 0…n−1.",
      "minGroupSize": 2,
      "maxGroupSize": 1000
    },
    "pairCollisionProbability": 0.002744051480544937,
    "groupSizes": [
      {
        "groupSize": 10,
        "classic": 0.11694817771107766,
        "historical": 0.1171186956039264
      },
      {
        "groupSize": 23,
        "classic": 0.5072972343239854,
        "historical": 0.5078220129591833
      },
      {
        "groupSize": 30,
        "classic": 0.7063162427192687,
        "historical": 0.7068480569716223
      },
      {
        "groupSize": 50,
        "classic": 0.9703735795779884,
        "historical": 0.9705192874184908
      },
      {
        "groupSize": 100,
        "classic": 0.9999996927510721,
        "historical": 0.999999698137179
      }
    ],
    "crossingFiftyPercent": {
      "classic": 23,
      "historical": 23
    },
    "specificBirthday": {
      "roomSize": 23,
      "otherPeople": 22,
      "question": "At least one of 22 others shares one specified person’s birthday.",
      "classicAnyDate": 0.058571325264343346,
      "septemberNineHistorical": 0.06446545495387906,
      "septemberNineShare": 0.003024381276911257
    }
  }
}