Kvartiilit

Kvartiilit, or quartiles, are statistical values that divide an ordered data set into four equal parts. They summarize the distribution by locating the points below which a quarter of the data fall.

The three quartiles are Q1, Q2, and Q3: Q1 marks the 25th percentile (the value below which 25% of data lie); Q2 is the median (the 50th percentile); Q3 marks the 75th percentile. The interquartile range, IQR = Q3 − Q1, measures the spread of the middle half of the data.

Calculation methods: ranking data, optionally including interpolation. Common approach: Q2 is the median of all data; Q1 is the median of the lower half; Q3 is the median of the upper half. Some software uses interpolation or different halving rules, so quartile values may differ for small data sets.

Applications: box plots, descriptive statistics, and outlier detection. Depending on IQR, outliers are often defined as observations below Q1 − 1.5·IQR or above Q3 + 1.5·IQR.

Terminology: Kvartiilit is the Estonian term for quartiles; in English, the term is quartiles. They belong to the broader family of quantiles, which also include percentiles and deciles.