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How do we measure the variability of data if it varies each time it is recorded…and why is this important? Let’s say you have been given data from a sleep study that found that its participants slept about 7 hours each night over a month. Each night will not always equal exactly 7 hours; some nights might average less than 7 hours and some nights more than 7 hours. How can we measure this variance over the spread of the data? In this section, you learn how the measures of variability—variance and standard deviation—are calculated and when to use them. Keeping in mind examples like the sleep study will help you to think about other concrete applications of these measures as you work through this section.