2-7 Bar Graphs & Histograms
Pages 85-89
Indicators
D1 -Read and create graphs
D2 -Analyze how decisions about graphing affect
the graphical representation
What is the difference between a
bar graph and a histogram?
There are two differences
• The type of data that is presented
• The way they are drawn
– The difference in the way that bar graphs
and histograms are drawn is that the bars in
bar graphs are usually separated, in histograms the
bars are adjacent to each other.
– This is not always true however. Sometimes you see
bar graphs with no spaces between the bars but
histograms are never drawn with spaces between the
bars
Bar Graphs
Bar graphs are usually used to
display "categorical data", that is
data that fits into categories.
For example suppose that I
offered to buy donuts for six
people and three said they
wanted chocolate covered,
2 said plain and
one said with
icing & sprinkles.
Donuts
Chocolate
Plain
Sprinkles
Types
Histograms
Histograms are usually used to present
"continuous data", that is data that
represents measured quantity where, at least
in theory, the numbers can take on any value
in a certain range.
A good example is weight. If you measure the
weights of a group of adults you might
get and numbers between 90 lbs. and
240 lbs. We usually report our weight as
pounds or to the nearest 1/2 lb. but we
might do so to the nearest 1/10 lb.
depending on how accurate the scale is.
To make a histogram…
• Remember: A histogram is a
particular kind of bar graph.
• The data that is being
represented is in intervals…
• So…to organize your data, make a
• Frequency table first!
• Then transfer the organized data
to the histogram
Histogram example:
The weight data would then be collected
into categories to present a histogram.
Weights of adults
For Example:
40
# of people
30
20
10
90
I
150 I
Pounds
210
I
might be a histogram for weights (with the appropriate
scale on the vertical axis). Here the data has been
collected into categories of 30 lbs.
To sum it all up…
• A histogram is a particular kind of bar
graph.
• If you make a bar graph of the favorite
colors of children in Mrs. Flaherty's 4th
grade class (6 for pink, 8 for blue, 1 for
black, etc.) that doesn't qualify as a
histogram, because the colors don't
correspond to any numerical values, and
their order is arbitrary.
• A different kind of example: you could
make a bar graph of monthly returns on
your stock investment in January, February,
etc. Here, there IS a natural order to the
bars, but it still doesn't qualify as a
histogram.
To finish summing it all up…
• A histogram is a bar graph of
frequencies of different numerical
values within a population. The most
straightforward kind of example may
be a bar graph of the number of 4th
grade students by height: how many
between 4' and 4'1" between 4'1" and
4'2" between 4'2" and 4'3"
• Both axis use numbers!
Homework
Pg. 88 #14-16 and Pg. 569 #1-4