Time-weighted averages take into consideration not only the numerical levels of a particular variable, but also the amount of time spent on it. For instance, if workers are exposed to different doses of noise for different amounts of time, we might use time-weighted averages -- acknowledging the differences in the amounts of time spent exposed to different amounts of noise -- to determine a worker's average amount of sound exposure.
Multiply each value by its time weighting. For instance, if a worker is exposed to 86 dB of noise for 13 hours a week, 26 dB of noise for 23 hours a week, and 0 dB of noise for 4 hours a week, you would obtain 86 x 13, 26 x 23 and 0 x 4 (1118, 598, and 0 dB hours, respectively).
Sum the values that you obtained in Step 1. In this case, you will obtain 1716 dB hours.
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Add together the time weights to get the total weight. In this case, the total weight is 13 + 23 + 4 = 40 hours.
Divide the value in Step 2 by the total weights in Step 3, to obtain 1716 / 40 = 42.9 dB.