# What is length of observation?

The length of observation represents the magnitude, duration, or size of each observation period. Use the length of observation to convert the rate of occurrence into a form that best suits your situation.

For example, if each sample observation counts the number of events in a year, a length of 1 would represent a yearly rate of occurrence while a length of 12 would represent a monthly rate of occurrence.

Minitab uses total occurrences, N, and the length of observation to calculate the rate of occurrence. For example, inspectors inspect the number of defects in a box of towels. A towel can have more than one defect, such as 1 tear and 2 pulls (3 defects). Each box contains 10 towels, they sample 50 total boxes, and they find a total of 122 defects.
• The total occurrences is 122, because they found 122 defects.
• The sample size (N) is 50 because they sampled 50 boxes.
• To determine the number of defects per towel, inspectors use a length of observation of 10 because there are 10 towels per box. If they wanted the number of defects per box, the length of observation would be 1.
• The rate of occurrence is (Total occurrences / N) / (length of observation) = (122/50) / 10 = 0.244. So on average, each towel has 0.244 defects.
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