Our example clock would be a very good crystal clock because the rate variations as shown in the 4th column are small. Nevertheless, the rate shows a systematic increase of 20ms in 14 days, i.e., the clock has a noticeable drift. The widely adopted and by far the most simple measure of clock stability is the co called Allan Variance , internationally known as two sample sigma . Form the squares of the second differences, add them, divide by 2 times the number of terms and form the square root. Such stability measures are also often expressed in relative terms, i.e., as parts per million, etc.
More information is available in literature upon request.