Both stopwatches were low cost devices; stopwatch A had a suggested
list price of about $25 USD, and stopwatch B sells for about $55 USD. The
laboratory temperature during the measurement was about 23 °C ± 1 °C. The
graph shows the Allan deviation (ADEV) of each stopwatch for averaging
times ranging from 0.5 days to 7 days. ADEV is a commonly used statistic for
estimating frequency stability [19]. It differs from the conventional standard
deviation because it does not use the average frequency as a point of reference.
Instead, it compares the frequency offset of the DUT during each measurement
period with its frequency offset during the previous measurement period. By
doing so, it reveals how the frequency of an oscillator changes over time due to
effects such as aging.