If you have an even number of points, the median lies between the points closest to the center. In the above chart, at the beginning of the time period, student satisfaction with the cafeteria was fairly level. The slight changes from 1980-89 are normal and represent common cause variation , the everyday variation that occurs in any process. To identify the causes of shifts and trends, a team needs to examine what conditions may have changed just before the shift or trend occurred. In the example above, the shift may have occurred right after the university contracted with a new food service.
Clusters are also a critical pattern to examine when analyzing time plots.
Clusters are created whenever a line connecting two points crosses the median. If a time plot has too few clusters, the data may represent a cycle or long-term trend . Too many clusters may indicate overcompensation or poor data collection . Circling the clusters provides a visual prompt for analyzing the pattern created. In the vacation schedule chart, June through September and December and January are consistently the most popular.