A histogram is a picture (a plot) which shows you the full range of values measured for a sample. The biggest values lie on the right, and the smallest values are found on the left. Individual measurements are grouped into bins of a set width; the higher the y-value, or number of Counts, for a bin, the more measurements it contains. By reading the plot, you can assess the mean of the sample (the average value) and the standard deviation, the degree to which the individual measurements differ from the average value.

One way of interpreting a histogram is by superimposing a normal curve, one which shows you a smooth pattern matching the overall distribution of values. A normal curve has the shape of a bell or an old-fashioned hat, having a peak in the center and sloping down on either side.

If the rare largest values of the sample are on the slope to the right, and the rare smallest values in the sample are on the slope to the left, where do you think the most common, mean value is found on the normal curve?

You can find the standard deviation of a sample by visualizing two vertical lines, one on either side of the mean value of your histogram. Place them evenly so that the area between the lines is two-thirds of the total area under the normal curve. The distance between the mean value and either of these vertical lines will be one standard deviation.