Bar Charts and Histograms
Like dotplots,
bar charts and histograms are used to compare the sizes of different groups.
Bar Charts
A bar chart is made up of columns plotted on a graph.
Here is how to read a bar chart.
 The columns are positioned over a label that represents a
categorical variable.
 The height of the column indicates the size of the group
defined by the column label.
The bar chart below shows average household income for the four
"New" states  New Jersey, New York, New Hampshire, and
New Mexico.
Income 
$36,000
$24,000
$12,000








New Jersey 
New Hampshire 
New York 
New Mexico 

The chart shows that per capita income is highest in New Jersey; lowest, in New Mexico.
Histograms
Like a bar chart, a histogram is made up of columns plotted on a graph.
Usually, there is no space between adjacent columns.
Here is how to read a histogram.
 The columns are positioned over a label that represents a continuous,
quantitative variable.
 The column label can be a single value or a range of values.
 The height of the column indicates the size of the group
defined by the column label.
The histogram below shows per capita income for five
age groups.
Income  $40,000 $30,000 $20,000 $10,000  
     
 2534  3544  4554  5564  6574 
You can see from the chart that per capita income is greatest in the 45 to 54 age group.
The Difference Between Bar Charts and Histograms
Here is the main difference between bar charts and histograms.
With bar charts, each column represents a group defined by a
categorical variable; and with histograms, each column represents
a group defined by a continuous, quantitative variable.
One implication of this distinction: It can be appropriate to
talk about the
skewness
of a histogram; that is, the tendency of the observations to fall
more on the low end or the high end of the X axis.
With bar charts, however, the X axis does not have a low end or
a high end; because the labels on the X axis are categorical  not
quantitative. As a result, it is not appropriate to comment on
the skewness of a bar chart.
Test Your Understanding
Problem 1
Consider the histograms below.
Which of the following statements are true?
I. Both data sets are symmetric.
II. Labels on the X axis are quantitative.
(A) I only
(B) II only
(C) I and II
(D) Neither is true.
(E) There is insufficient information to answer this question.
Solution
The correct answer is (C). Both histograms are mirror images around
their center, so both are
symmetric.
With bar charts, the labels on the X axis are categorical;
with histograms, the labels are quantitative. Both of these
charts are histograms. Therefore, their labels are quantitative.