Section 2.2
Graphical Displays of Distributions

Dot Plots

Histograms: uses bars to show quantity of
cases within a range of values

Stem-and-leaf or Stemplots: use actual #s
to show the quantity of cases within a range

Bar Charts: for categorical data

Quantitative: counts of how many and
measurements of how much

Categorical: groups cases into categories

Refer to page 43 for table of mammals
 Gestation, Longevity, Speed are quantitative
 Wild (Y or N), Predator (Y or N) are categorical

Relatively small # of values to plot

You want to see individual values

You want to see shape of the distribution

You want to compare the distribution of a
small number of groups
• Similar to dot plots in that they show the quantity of data at certain values.
• The difference is that the bars show how many data values there are within intervals
called bins. Notice that each bin has a width of 5 (Xscale).

Refer to page 45 to see some examples
regarding characteristics of mammals.

What characteristics of the graph do you
notice?

Do histograms show quantitative data or
categorical data?

Using your calculator to show a histogram
 Enter the data for “speed” into L1 in your calc.
▪ Stat, Edit







2nd Stat plot to turn on a plot of a list
Select the histogram, L1
Window and set Xmin/max – min to max+ values (0 – 80)
Set Ymin/max – 0 to your estimate of the largest frequency
Set Xscale – will set the bin sizes (5)
Set Yscale – will set scale for frequency along the y-axis (1)
Graph
▪ Change the Xscale to 10 - What happens??

To change a Frequency Histogram into a
Relative Frequency Histogram:
 Simply divide the frequency for each bar by the
total number of values in the data set.
 These will now be proportions.
 Show these proportions of the total on the
vertical axis.
 Now instead of actual frequencies being shown,
you have proportions of the total being shown.

Frequency Hist.
Relative Frequency



Notice the vertical (Y-axis):
Notice the shapes – they are the same
Describe the shape of the distribution, find the
median, lower and upper quartiles.


You have large number of values to plot
You don’t need to see individual values.
 In other words ranges (bins) are fine


You want to see the general shape of the
distribution.
You can use a calculator or software to create
the plots for you.


P8 Make histograms of data for average
longevity and then max longevity from
display 2.24. Compare these distributions
using shape, center, spread, gaps, clusters
and outliers. Why do these occur?
Homework: Create a sentence or word that
helps to remember S C S G C O. We will vote
on the best and most creative for extra credit.
YOU WILL LIVE IN INFAMY!!!!


P8 Convert the frequency histograms into
relative frequency histograms.
P10 Using Rel Freq Hist from 2.28 on p. 47.
 Estimate the proportion of countries with a life
expectancy of less than 50 years.
 Describe the shape of the distribution
 Estimate the median of this distribution

Page 53 E16, 17, 18, 20, 21

AP add #22