The data and the interquartile range are displayed on the dot plot below. The median is the mean of the two central values, 18.7 and 18.8. The highest number in the data set is 10. The range is the number when you subtract the highest number and the lowest number. ![]() It is recommended that a graph of the distribution is used to check the appropriateness of the interquartile range as a measure of spread and to emphasise its meaning as a feature of the distribution. Quartiles tell us about the spread of a data set by breaking the data set into quarters, just like the median breaks it in. The mean is the average number of the data set (to find it, you have to add up all of the numbers (sum) and then divide it by how many numbers there are). The interquartile range is more useful as a measure of spread than the range because of this stability. The interquartile range is a stable measure of spread in that it is not influenced by unusually large or unusually small values. It is recommended that, for small data sets, this measure of spread is calculated by sorting the values into order or displaying them on a suitable plot and then counting values to find the quartiles, and to use software for large data sets. It is calculated as the difference between the upper quartile and lower quartile of a distribution. A measure of spread for a distribution of a numerical variable which is the width of an interval that contains the middle 50% (approximately) of the values in the distribution.
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