Before we get into the discussion about confidence intervals, we need to address the concept of “unknown population parameter”.

What is a parameter?

A parameter tells us something about a population (while a “statistic” tells us something about a sample). The population parameter is thus basically a measure of any given population.

The parameter is a fixed value, i.e. it does not vary.

Example Examples of population parameters are: the mean height of Swedish men, the average intelligence score in 12-year olds, or the mean number of children among highly educated people.

Why is it unknown?

We seldom have information about the entire population, generally only for a part of it (a sample). In that case, the population parameter is unknown.

Simply put, a confidence interval is a range that includes the “unknown population parameter”.