A graphical representation of a distribution function by means of rectangles whose widths represent intervals into which the range of observed values is divided and whose heights represent the number of observations occurring in each interval. For example, if measurements of length have been taken for bronze spearheads from one particular area and period, the measurements are represented by marking off intervals of lengths on the horizontal axis, and counting the number of spearheads falling into each division. These numbers are marked off on the vertical axis. In order to compare one set of data with another, or others, a cumulative version of the histogram may be used, where the succeeding values are added to the preceding: these are called cumulative frequency polygons, and are useful for comparative work, but are difficult to use if single histograms need to be extracted. A useful way to assess the density of rocks is to make a histogram plot of the statistical range of a set of data. The representative value and its variation can be expressed as follows: (1) mean, the average value, (2) mode, the most common value (i.e., the peak of the distribution curve), (3) median, the value of the middle sample of the data set (i.e., the value at which half of the samples are below and half are above), and (4) standard deviation, a statistical measure of the spread of the data (plus and minus one standard deviation from the mean value includes about two-thirds of the data).