How to Build a Confidence Interval Calculator

// Statistics functions
function mean(values) {
  return values.reduce((sum, v) => sum + v, 0) / values.length;
}

function standardDeviation(values) {
  const avg = mean(values);
  const squareDiffs = values.map(v => Math.pow(v - avg, 2));
  return Math.sqrt(mean(squareDiffs));
}

function confidenceInterval(values, confidenceLevel = 0.95) {
  const n = values.length;
  const avg = mean(values);
  const stdDev = standardDeviation(values);
  const zScore = getZScore(confidenceLevel); // 1.96 for 95%
  const margin = zScore * (stdDev / Math.sqrt(n));

  return {
    mean: avg,
    lower: avg - margin,
    upper: avg + margin,
    margin
  };
}