How To Do Bar Graphs
Graphing Rules
Introduction
A bar chart or bar graph
is a chart with rectangular bars
with lengths proportional to the
values that they represent. Bar
charts are used for comparing two or
more values that were taken over
time or on different conditions,
usually on small data sets. The bars
can be horizontally oriented (also
called bar chart) or vertically
oriented (also called column chart).
Sometimes a stretched graphic is
used instead of a solid bar. It is a
visual display used to compare the
amount or frequency of occurrence of
different characteristics of data
and it is used to compare groups of
data.
Rules
for Graphing All lines must be equal
distance apart. All units equal.
When numbering and labeling, you must use the same movement between each
line (for example: 2, 4, 6, 8) Always
label the line
on the y
(vertical) axis. The y axis is
reserved for the responding or dependent variable. The
x axis or horizontal
axis can be labeled between the lines. The
x axis is reserved for the manipulated
or independent variable. Your graph
must have a title. The graph title gives
an overview of the information being presented in the graph.
The title is given at the top of the graph.
Examples
One Independent and One Dependent Variable
See
more examples

Simple Bar Graph
Here the "Factory" is our
independent or manipulated variable, since there is no unit of
measurement for factories and no 'order' to the
factories, the independent variable is nominal.
The dependent or responding variable is scalar, measured in
defects/1,000 cars. Since the scalar dependent
variable has a natural zero point (i.e.,
absolute or ratio), all of the bars are anchored
to the horizontal axis, giving a common point of
measurement.

Bar graphs can be shown
with the responding or dependent variable on the horizontal
scale. This type of bar graph is typically
referred to as a horizontal bar graph. Otherwise
the layout is similar to the vertical bar graph.
Note in the example above, that when you have
welldefined zero point (ratio and absolute
values) and both positive and negative values,
you can place your vertical (manipulated or independent
variable) axis at the zero value of the
responding or dependent variable scale. The negative and
positive bars are clearly differentiated from
each other both in terms of the direction they
point and their color.

Range bar graphs
represents the responding or dependent variable as interval
data. The bars rather than starting at a common
zero point, begin at first dependent variable
value for that particular bar. Just as with
simple bar graphs, range bar graphs can be
either horizontal or vertical. Notice in the
horizontal example above, a reference line is
used to indicate a common key dependent variable
value.

Histograms are similar
to simple bar graphs except that each bar
represents a range of independent variable
values rather than just a single value. What
makes this different from a regular bar graph is
that each bar represents a summary of data
rather than an independent value. For this type
of graph, the dependent variable is almost
always a scalar scale representing the count, or
number, of how many of a sample fall within each
range of the independent variable. In the
example above, the sample is all the females in
the U.S. The independent variable is age, which
as been grouped into ranges of 5 years each. You
should try and keep the ranges for each bar
uniform (5 years in this case), with the
exception possibly being the first and/or last
range.
Two (or more) Independent and One Dependent
Variable

Here, we have taken the
same graph seen above and added a second
independent variable, year. The initial
independent variable, factory, is nominal. The
second independent variable, year, can be
treated as being either as ordinal or scalar.
This is often the case with larger units of
time, such as weeks, months, and years. Since we
have a second independent variable, some sort of
coding is needed to indicate which level (year)
each bar is. Though we could label each bar with
text indicating the year, it is more efficient
to use color. We will need a legend to explain
the color coding scheme. Note that all of the
bars for each level of factory are touching each
other, indicating visually that they are grouped
together.

Another alternative for
a bar graph with two independent variables is to
have the bars stacked rather than sidebyside.
This arrangement is useful when the summation of
all the levels of the second independent
variable is as or more important than the values
for each level. In the upper example, it is very
easy to read the summed weight of all of the
different materials in each sample. There are,
however, tradeoffs. The stacking of the bars
means there is no common baseline for the
individual bar elements, making it hard to make
direct comparisons for the subcategories. For
example, it is hard to compare the iron content
of the three samples. A particularly powerful
use for the composite bar graph is when the sum
of all the dependent variable values for each
bar is the same, such as when the values are a
fraction of a whole. In the bottom example, the
sum of the three different types of fats will
always equal 100 percent. With this layout it is
easier to see the relative portions, if not the
absolute values, of a particular fat type across
oils.
Specific tips for bar graphs on Excel
 Vertical bar graphs are called column
graphs in Excel
 Horizontal bar graphs are called bar
graphs in Excel
 The clustered subtype will do single
bar and grouped bar graphs
 The stacked subtype will do composite
bar graphs
 Range bars cannot easily be done in
Excel without additional custom graph types
loaded
 Doubleclicking on the bars will allow
you to set your preferences for bar display.
Under the Options tab you can set the ratio
of the bar width to gap between bars

Susan Augustyn
Room 31


