Читать книгу Medical Statistics - David Machin - Страница 49

The quartiles, namely the lower quartile, the median and the upper quartile, divide the data into four equal parts using three cut‐points; that is there will be approximately equal numbers of observations in the four sections (and exactly equal if the sample size is divisible by four and the measures are all distinct). The quartiles are calculated in a similar way to the median; first order the data and then count the appropriate number from the bottom. The lower quartile is found by ranking the data and then taking the value below which 25% of the data sit. The upper quartile is the value above which the top 25% of the data points sit. The interquartile range is a useful measure of variability and is the range of values that includes the middle 50% of observations and is given by the difference between the lower and upper quartiles. The interquartile range is not vulnerable to outliers, and whatever the distribution of the data, we know that 50% of them lie within the interquartile range.