Measures of dispersion help to describe the extent of variability within a data set or the spread of data. The four most commonly used measures of dispersion are range, interquartile range, variance and standard deviation.
Range is the difference between the extremes of the data set. The range is easy to calculate; however, it is very sensitive to outliers.
Interquartile range is the difference between the upper and lower quartiles (75th percentile – 25th percentile). It is useful for looking at the central data and is not as sensitive to extremes of data as the range.
Variance describes how close the data distribution is to the middle of the distribution. It is calculated by taking the average squared difference of the data from the mean divided by the total number of data points.
X = each data point; M = mean of the data set; N = total number of data points
Standard deviation measures the spread of the data from the mean. It is the most commonly used measure of dispersion. It is calculated by taking the square root of the variance. It is most useful in data with a normal distribution.
X = each data point; M = mean of the data set; N = total number of data points
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