1.2 - Displaying quantitative data with

graphs(Histograms)

HistogramsThe most common graph of quantitative data.

Classes: the intervals along the bottom axis. These need to be of equal width

Frequency: the count of individuals of a class occurring

Relative frequency: the percent of the individuals in a class

(this is more useful, especially when you are comparing two sets of data with an unequal total

of individuals)

Construct 2 histograms.One displaying the frequency and one

displaying the relative frequency.

The following table presents the average points scored per game (PTSG) for the 30

NBA teams in the 2009-2010 regular season.

101.7 99.2 95.3 97.5 102.1 102

106.5 94 108.8 102.4 100.8 95.7

101.7 102.5 96.5 97.7 98.2 92.4

100.2 102.1 101.5 102.8 97.7 110.2

98.1 100 101.4 104.1 104.2 96.2

Steps for constructing a histogram

1st - divide the range of data into class of equal width.

2nd - find the count and percent of individuals in each class.

3rd - label and scale your axes

4th - draw your histogram

1st stepWhat is the range of our data?

What would be a good class size to choose?

What are the classes?

2nd StepFill in a frequency table and a relative frequency

table.Frequency

TableClass Count

Relative Frequency Table

Class Percent

3rd step

PTSG

Frequ

en

cy

PTSGR

ela

tive F

req

uen

cy

4th step

PTSG

Frequ

en

cy

PTSGR

ela

tive F

req

uen

cy

Describe the data of the average points per game in the 2009-2010 NBA

season.

Don’t forget your “SOCS!”The data

appears to be _______

with a peak of _____ .

The _____ appear to be any outliers.

The center of the data

occurs around ______

The histogram shows that

the percent of points per

games ranged from

__________

How does class size effect the shape of the

histogram?

www.whfreeman.com/tps4e

Last Pieces of advice about histograms

1. Don’t confuse histograms and bar graphsHistograms are for quantitative dataBar graphs are for categorical data

4. Just because a graph looks nice, doesn’t mean it’s necessarily a

meaningful display of data. (Excel is a terrible tool to use for

statistical graphs)

3. Use percents instead of counts when comparing distributions with different numbers of

observations.

2. Don’t use the counts or percents as the data. Use the data to find the counts and percents for your

graph.