In chapter 7 we discussed the Amplitude-PHase-Time (APHT) plots, a form of polar and Bode plots, which are used for trending vector data. Besides vector data, there are many other parameters that we would like to trend: vibration levels, position data, process data, or any other parameter that can be useful for machine condition monitoring. The trend plot shows changes in and the rate of change of parameters that may signal a developing or impending problem in a machine. This information can be used to set limits or thresholds for action.