Trend Lines

Trend lines, also called lines of progress or celeration lines, are drawn through charted data to summarize the relationship between two variables. The two variables usually represent changes in performance over time (see Figure 1, panels A and B), but the relationship between any two variables can be summarized with trend lines (panels C and D). Trend lines have been used to describe patterns within phases of a study and to predict values beyond the immediate data set (panels A and B). Because accepted “theoretical” or “mathematical” justifications do not exist for predicting values (extrapolating) beyond the data set used to estimate the trend, experience and empirically demonstrated success must be used in deciding whether the data are orderly enough to warrant predictions. [Page ...