The average is the middle value of a data set. The average can also be referred to as the central tendency. The most common method of arriving at an average for a data set is to add the individual values of the data and then divide that total by the number of data points in the set. This average number is also known as the mean.
The time weighted average allows you to put value (or weight) on certain aspects of a data set. In the workplace, a time weighted average is used to calculate the exposure that employees encounter to hazardous substances such as chemicals, fumes and vapors. The time is weighted to an 8-hour working day and the average substance level and time spent in the hazardous area are used in the calculation.