Confidence Interval
A range of values, derived from sample statistics, that is likely to contain the true population parameter.
What is Confidence Interval?
A confidence interval (CI) provides a range of plausible values for an unknown population parameter (like a mean or difference in means). It is associated with a confidence level (e.g., 95%), which represents the frequency with which the interval would contain the true parameter if the experiment were repeated indefinitely.
Why Confidence Interval Matters
Unlike a single point estimate (like a sample mean) or a p-value, a confidence interval provides both an estimate of the effect size and a measure of its precision or uncertainty.
How to Interpret
If a 95% confidence interval for a difference between two groups does not include zero, the difference is statistically significant at the p < 0.05 level.
Example
A poll might state that 60% of voters support a policy, with a 95% confidence interval of [56%, 64%]. This means we can be highly confident the true support level in the population lies within that range.
Common Mistakes
- Stating there is a 95% probability that the true parameter falls within this specific computed interval (in frequentist statistics, the parameter is fixed; it is the interval that either contains it or doesn't).