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A confidence interval (CI) gives a range of plausible values for a population parameter based on a sample. A 95% CI means: if we repeated this study many times, 95% of the resulting intervals would contain the true population value.

Trinn-for-trinn guide

1. 1Calculate the sample mean (x̄) and standard deviation (s)
2. 2Determine the z* critical value for your confidence level (95% → z* = 1.96)
3. 3Calculate standard error: SE = s / √n
4. 4Margin of error: ME = z* × SE
5. 5CI = (x̄ − ME, x̄ + ME)

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