What is a confidence interval?
A confidence interval gives a plausible range for a true population value based on sample data.
The common misconception about CIs
A 95% interval does not mean the true value has a 95% probability of being inside this exact interval.
Z-interval vs t-interval
Use a z-interval when population standard deviation is known. Use a t-interval when you estimate spread from the sample.
How to interpret confidence intervals
Wider intervals mean more uncertainty. Larger samples and lower confidence levels produce narrower intervals.