Chapters 10,11, and 12 (Student)
1
Graphs, Good and Bad
•
Recall the definition of statistics:
•
Statistics is the science of collecting,
classifying
,
presenting
, and interpreting data.
Types of Variables
•
Quantitative variable
– takes numerical values for
which arithmetic operations make sense.
Examples: amount of money, number of children, distance
•
Categorical variable
– places an individual into one of
several groups or categories
Examples: gender, race, academic major, zip code
Distribution of a Variable
•
The distribution
of a variable tells us what values it
takes and how often each value occurs.
•
How do we describe distributions?
Tables or graphs
Numerical summaries
•
There are several graphical (pictorial) ways to describe/display
distributions.
The method used is determined by the type of
variable (data) and the idea to be presented.

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Chapters 10,11, and 12 (Student)
2
Tables and Graphs for Categorical Data
• Frequency
(count) – the number of times a value of a
variable occurs in the data
•
Relative Frequency
– the proportion (fraction or
percent) of all observations that have a given value
Example: Table of Exam 1 Grades
4%
0.04
7
F
10%
0.10
20
D
12%
0.12
24
C
26%
0.26
51
B
48%
0.48
95
A
Percentage
Relative
Frequency
Frequency
Grade
Graphs for Categorical Data
•
Basic graphs for summarizing categorical variables
(data) are pie charts and bar graphs.
•
Pie chart
– shows the amount of data that belongs to
each category as a proportional part of a circle.