Page 17 - 2024-bfw-starnes-TPS7e-SE proofs.indd

4 UNIT 1 Exploring One-Variable Data

Most data tables follow this format: each row describes an individual, and each
column contains the values of a variable. Sometimes the individuals in a data set
are called cases or observational units .

DEFINITION Individual, Variable
© 2024 BFW Publishers PAGES NOT FINAL - For Review Purposes Only - Do Not Copy
An individual is an object described in a set of data. Individuals can be people,
animals, or things.
A variable is a characteristic that can take different values for different individuals.

For the ACS data set, the individuals are households. It is important to note that
household is not a variable in this data set — the numbers in that column of the data
table are just labels for the individuals. The variables recorded for each household are
region, people in household, time in their current dwelling, survey response mode,
household income, and whether the dwelling has internet access. Region, time in
dwelling, response mode, and internet access status are categorical variables. People
in household and household income (in dollars) are quantitative variables.

DEFINITION Categorical variable, Quantitative variable
A categorical variable takes values that are labels, which place each individual into
a particular group, called a category.
A quantitative variable takes number values that are quantities — counts or
measurements.

Not every variable that takes number values is quantitative. Zip code is one
caution
example. Although zip codes are numbers, they are neither counts nor measure-
ments of anything. Instead, they are simply labels for a regional location, making
zip code a categorical variable. Time in dwelling from the ACS data set is also
a categorical variable because the values are recorded as intervals of time, such
as 2–4 years. If time in dwelling had been recorded to the nearest year for each
household, this variable would be quantitative.
The variable “year” is often treated as categorical. But it depends on how the data
are being used. Consider a data set about cars, in which one of the variables recorded
is model year. If we want to know what percentage of cars on the road are 2022 mod-
els, we treat year as categorical. If we want to know the average age of cars on the road,
we would convert model year to age (in years) and treat this variable as quantitative.

EXAMPLE census at school Skill 2.A
Individuals and variables

PROBLEM: Census at School is an international project that collects

data about primary and secondary school students using surveys. Since
its launch in 2000, students from Australia, Canada, Ireland, Japan, New
Zealand, South Africa, the United Kingdom, and the United States have
taken part in the project. We selected a random sample of 50 U.S. high
school students who completed the survey in a recent year. The table displays
data on some of the survey questions for the first 10 students in the sample.
EF Volart/Moment/Getty Images

© 2024 BFW Publishers PAGES NOT FINAL - For Review Purposes Only, all other uses prohibited - Do Not Copy or Post in Any Form.

02_StarnesTPS7e_40934_un01_p1_001_086_6pp.indd 4 13/09/23 5:36 PM