1.1 Types of Statistical Studies
Research questions about a population:
Type of Research Question
Examples
What is the average number of hours that
Make an estimate about the population (often an
community college students work each week?
estimate about an average value or a proportion
What proportion of all U.S. college students are
with a given characteristic)
enrolled at a community college?
Is the average course load for a community
Test a claim about the population (often a claim
college student greater than 12 units?
about an average value or a proportion with a given
Do the majority of community college students
characteristic)
qualify for federal student loans?
In community colleges, do female students have
Compare two populations (often a comparison of
a higher GPA than male students?
population averages or proportions with a given
Are college athletes more likely than nonathletes
characteristic)
to receive academic advising?
Is there a relationship between the number of
hours high school students spend each week on
Investigate a relationship between two variables in
Facebook and their GPA?
the population
Is academic counseling associated with quicker
completion of a college degree?
Cause and Effect
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Does cell phone usage increase the risk of developing a brain tumor?
Does drinking red wine lower the risk of a heart attack?
Does playing violent video games increase aggressive behavior?
Does sex education lower the incidence of teen pregnancy?
In these types of studies, one variable is the explanatory variable, and the other is
the response variable. To establish a cause-and-effect relationship, we want to make
sure the explanatory variable is the only thing that impacts the response variable.
We therefor get rid of all other factors that might affect the response. Then we
manipulate the explanatory variable. Our goal is to see if it really does affect the
response.
An experiment intentionally manipulates one variable in an attempt to cause an
effect on another variable. The primary goal of an experiment is to provide evidence
for a cause-and-effect relationship between two variables. But the experiment has to
be well-designed to provide convincing evidence of a cause-and-effect relationship.
An observational study observes individuals and measures variables of interest. The
main purpose of an observational study is to describe a group of individuals or to
investigate an association between two variables.
Observational Study:
Do the majority of college students listen to music while they study? Do the majority of
college students believe that listening to music improves their learning?
To investigate these questions, the statistics students conduct a survey in their other
classes. They ask these two questions:

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Do you listen to music while you study?
Do you think listening to music improves your concentration and memory?
Experiment:
Does listening to music improve students’ ability to quickly identify information?
To investigate this question, the instructor uses word-search puzzles. She divides
the class into two groups. Students on one side of the room do a word puzzle for 3
minutes while listening to music on an iPod. Students on the other side of the room
do a word puzzle for 3 minutes without music. The instructor calculates the average
number of words found by each group.
Let’s Summarize
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There are four steps in a statistical investigation:
o Ask a question that can be answered by collecting data.
o Decide what to measure, and then collect data.
o Summarize and analyze.
o Draw a conclusion, and communicate the results.
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There are two types of statistical research questions:
o Questions about a population
o Questions about cause-and-effect

To answer a question about a population, we select a sample and conduct an
observational study. To answer a question about cause-and-effect we
conduct an experiment.
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There are two types of statistical studies:
o Observational studies: An observational study observes individuals
and measures variables of interest. We conduct observational studies
to investigate questions about a population or about an association
between two variables. An observational study alone does not provide
convincing evidence of a cause-and-effect relationship.
o Experiments: An experiment intentionally manipulates one variable in
an attempt to cause an effect on another variable. The primary goal of
an experiment is to provide evidence for a cause-and-effect
relationship between two variables.

In statistics, a variable is information we gather about individuals or objects.

When we investigate a relationship between two variables, we identify an
explanatory variable and a response variable. To establish a cause-and-effect
relationship, we want to make sure the explanatory variable is the only thing
that impacts the response variable. Other factors, however, may also
influence the response. These other factors are called confounding variables.
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The influence of confounding variables on the response variable is one of the
reasons that an observational study gives weak, and potentially misleading,
evidence of a cause-and-effect relationship. A well-designed experiment
takes steps to eliminate the effects of confounding variables, such as random
assignment of people to treatment groups, use of a placebo, and blind
conditions. For this reason, a well-designed experiment provides convincing
evidence of cause-and-effect.