tion 1.3. Circuit errors (1.3) Figure 1.4 Illustration of deviation from the average This technique is used to find the average of quantities which follow a geometric progression or exponential law, such as rates of changes. Its advantage is that it takes into account all the numbers, interquartile range, which is the difference between the first and but is not unduly influenced by extreme values. third quartile numbers, or the quartile deviation which is half this value. 1.3.4 Harmonic mean The quartile deviation is easy to use and is not influenced by extreme values. However, it gives no indication of distribution The harmonic mean of n numbers X1, X2, X3, Xn is given by between quartiles and covers only half the values in a series. Equation 1.4. 1.4.2 Mean deviation (1.4) This is found by taking the mean of the differences between each individual number in the series and the arithmetic mean, or median, of the series. Negative signs are ignored. This averaging method is used when dealing with rates or speeds For a series of n numbers X X2, having an arithmetic mean of x the mean deviation of the series is given by Equation 1.7. or prices. As a rule when considering items such as A per B, if the figures are for equal As then the harmonic mean is used, but if they are for equal Bs then the arithmetic mean is used. So if a plane flies (1.7) over three equal distances at speeds of 5m/s, 10m/s and 15m/s the mean speed is given by the harmonic mean as in expression 1.5. (1.5) 8.18m/s The mean deviation takes into account all the items in the series but it is not very suitable since it ignores signs. If however, the plane were to fly for three equal times, of say, 20 1.4.3 Standard deviation seconds at speeds of 5m/s, 10m/s and 15m/s, then the mean speed would be given by the arithmetic mean as in expression 1.6. This is the most common measure of dispersion. For this the arith- metic mean must be used and not the median. It is calculated by 5 + 10 + 15 (1.6) 10m/s squaring deviations from the mean, SO eliminating the