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Warm-Up
Homework Questions
Using Studies Wisely
Section 4.3
Students often confuse the two types of
inferences we can make: inferences about
a population and inferences about cause
and effect.
 What type of inference/conclusion can be
made from a particular study? The answer
depends on the design of the study.

Scope of Inference

In a census, the individuals who respond
are chosen at random from the population
of interest. Random sampling avoids bias
and produces trustworthy estimates of the
truth about the population. The Census
Bureau should be safe making an
inference about the population based on
the results of the survey.
Example

In a sleep deprivation experiment, subjects
were randomly assigned to the sleep
deprivation and unrestricted sleep
treatments. Random assignment helps
ensure that the two groups of subjects are as
similar as possible before the treatments are
imposed. If the unrestricted sleep group
performs much better than the sleep
deprivation group, and the difference is
statistically significant, it must be due to the
treatments. It that case, they can make
inferences about cause and effect.
Example
Were individuals randomly
selected?
Were individuals randomly assigned to groups?
Yes
No
Inference about
population: YES
Inference about
population: YES
Inference about cause
and effect: YES
Inference about cause
and effect: NO
Inference about
population: NO
Inference about
population: NO
Inference about cause
and effect: YES
Inference about cause
and effect: NO
Yes
No
Overview

The association is strong.

The association is consistent.

Larger values of the explanatory variable are
associated with stronger responses.
◦ The association between smoking and lung cancer is very
strong.
◦ Many studies of different kind of people in many
countries link smoking to lung cancer.
◦ People who smoke more cigarettes per day or who
smoke over a longer period of time get lung cancer more
often.

The alleged cause precedes the effect in time.

The alleged cause is plausible.
◦ Lung cancer develops after years of smoking.
◦ Experiments with animals show that tars from cigarette
smoke do cause cancer.
Criteria for Causation

On Part B of your AP Questions – look at
number 5! This deals with these topics!
Homework