Defects occur at specific frequencies in relation to the running speed of the equipment. Most vibration analysis software will allow these specific frequencies (bands) to be measured and trended over time. Trending this information will help identify problems as they occur in your equipment. This results in more accurate analysis of equipment problems that will help determine the severity and repair urgency of the problems identified.
For example, if the vibration trend is increasing slowly, then the failure may not be progressing rapidly. However, a sharp increase in a specific vibration trend over time indicates that a defect may have developed and failure is more imminent.