The Imperial Council of Agricultural Research India a Handbook of Statistics for use in Plant Breeding and Agricultural Problems

The mid-point of each class is taken as the class value and the frequency of the class consists of the laumber of variates which fall within the limits of the elhss. [...] The mean is the arithmetic average.and is the result obtained when the sum of the measurements of the items in a sample is divided by the number of items in the sample. [...] 3.—The normal curve and curves for values of (a + br."12 HANDBOOK OF STATISTICS FOR USE IN The slope of the curve is an indication of the amount of variability in the sample and the width at the point of greatest breadth indicates the range of variability. [...] The steeper the slope of the curve the shorter is the distance between a quartile and the mean and hence the distance between the quartile and the mean may be used as..ameasure of variability. [...] Of course no curve based on measuir ments of a biological variable will exhibit the perfect symmetry of the normal curvbut if the sampling has been adequate and the population is uniform the departure from symmetry is not such as to preclude the application of the statistical principles of the normal curve to the solution of such problems.