Histogram

A Histogram is a graphical display of the data in a table of frequencies. What should be plotted is the [frequency density]?. For example, we might have a collection of data for which the frequency table is as follows:

From	To	Frequency	Density
0	9	2	0.2
10	19	3	0.3
20	29	5	0.5
30	39	8	0.8
40	49	6	0.6
50	59	1	0.1
60	69	3	0.3

The histogram corresponding to these data would look like:

0.8                #####
0.7                #####
0.6                ##########
0.5           ###############
0.4           ###############
0.3      ####################     #####
0.2 #########################     #####
0.1 ###################################
    0---10---20---30---40---50---60---70

The distinction between a histogram and a [bar graph]? is that if we wish to find the total frequency of a range of values, we must consider the area under the graph (so between 30-39, we have a range of 10, and a frequency density of 0.8, giving the total frequency of 8)

Suppose we group the above data differently:

From	To	Frequency	Density
0	9	2	0.2
10	19	3	0.3
20	59	20	0.5
60	69	3	0.3

our histogram would look like this:

0.5           ####################
              ####################
0.4           ####################
              ####################
0.3      ##############################
         ##############################
0.2 ###################################
    ###################################
0.1 ###################################
    ###################################
    0---10---20---30---40---50---60---70