Statistical data

The set of all objects to be studied is the population, and a randomly chosen part of it is a sample; when checking everything is expensive or would destroy the objects, we use a sample survey. The number of objects in the sample is its size n. Non-numerical data such as colours or brands are called qualitative, those measured by numbers quantitative. Writing quantitative data in increasing order gives the variation series; its distinct values xᵢ are the variants, the number of times each occurs nᵢ is the frequency, and ωᵢ = nᵢ/n is the relative frequency, with n₁ + … + nₖ = n and the relative frequencies adding up to 1. Joining the points (xᵢ, nᵢ) of a frequency table by a broken line gives a polygon. When there are many values that rarely repeat, we group them into intervals of equal length and draw a histogram of rectangles whose bases are the intervals and whose heights are the frequencies.