Suppose you want to know what share of students in a school of 1 200 walk to school. You could ask everyone. Or you could ask 100 of them and use the answer to estimate the rest.

The words:

• Population: the whole group you want to learn about (all 1 200 students).
• Sample: the part of the population you actually measure (the 100 you asked).
• Parameter: a number that describes the population, such as the true share who walk. Usually unknown. Written p.
• Statistic: a number computed from the sample, such as the share of your 100 who walk. Written p̂ ("p-hat").

The whole idea of statistics fits in one sentence: we use a statistic to estimate a parameter we cannot afford to measure.

Sometimes the population can be measured completely. That is a census. A country's population count is an attempt at one. But measuring everyone is usually too slow, too costly, or impossible (you cannot test every light bulb until it burns out, or you would have none left to sell).