Download Section 1.3 ~ Types of Statistical Studies

Section 1.3 ~
Types of Statistical Studies
Introduction to Probability and Statistics
Ms. Young
Sec. 1.3
Objective
After this section you will understand
the differences between observational
studies and experiments, including the
selection of treatment and control
groups, the placebo effect, and
blinding.
Sec. 1.3
Types of Statistical Studies
Subjects – the people, animals (or other
living things), or objects chosen for the
study
If the subjects are people, they are
commonly referred to as participants
Sec. 1.3
Observational Study

Observe or measure characteristics of the
subjects without attempting to influence or
modify those characteristics
 Examples



~
Nielsen ratings; “people meters” measure the viewing
habits, but don’t influence what is being watched
Measuring weights; still considered observational even
though the researchers are interacting with the
participants because the person’s weight is not changing
based on the interaction
Opinion polls; considered observational as long as the
researcher doesn’t attempt to sway the participants
answers in any way
Sec. 1.3
Experiment

Observe the effects on the subjects
after applying a treatment
 Ex.
~ Vitamin C study; to determine
whether large doses of vitamin C help to
prevent colds

Considered an experiment because the
participants are actually given large doses (or
treatments) of vitamin C
Sec. 1.3
Example 1

Identify the study as observational or experimental
a. The Salk polio vaccine case study (p.8)

Experiment because researchers tested a treatment
(vaccine) to see whether it reduced the incidence of polio.
b. A poll in which people are asked for whom they plan to
vote in the next election.

Observational because it attempts to determine voting
preferences but does not try to sway votes.
Sec. 1.3
Variables of Interest
A variable is any item or quantity that
can vary or take on different values
 In a statistical study, variables of
interest are what we want to learn
about

 Example

~
Nielsen ratings; number of viewers watching the
Super Bowl would be the variable of interest
Sec. 1.3
Cause and Effect Cases

In cases where we think cause and
effect may be involved, we typically
subdivide the variables of interest into
two categories:
 Explanatory
variable – the variable that
may explain, or cause the effect

Ex. ~ the actual dose of vitamin C
 Response
variable – the variable that
responds to the changes in the explanatory
variable

Ex. ~ the number of colds that occurred
Sec. 1.3
Observational Studies

Retrospective study – an observational study
that uses data from the past
 Especially
valuable when the study would be
impractical or unethical to conduct


Ex. ~ studying the effects of consuming alcohol during
pregnancy
Prospective study – designed to collect data
in the future from groups that share common
factors
 Ex’s. ~
 How high school dropouts compare (socially, financially,
etc.) to high school graduates within 10 years of
graduation (or drop out date)
 The risk of heart disease among smokers
Sec. 1.3
Example 2

You want to know whether children born
prematurely do as well in elementary school as
children born at full term. What type of
study should you do?
 Retrospective
observational study; you would
collect data on past premature births (as well as
full term births) and compare their elementary
school performances
Sec. 1.3
Experiments

The need for controls
 Vitamin
C study – How can the researchers
know whether the subjects would have
gotten more colds without the vitamin C?

Must conduct the experiment with two (or
more) groups of subjects
Treatment group – the group that receives the
treatment being tested
 takes the large doses of vitamin C
 Control group – the group that does not receive the
treatment being tested
 Doesn’t take any doses of vitamin C

Sec. 1.3
Example 3

Look again at the Salk polio vaccine Case Study on
page 8. What was the treatment? Which group of
children constituted the treatment group? Which
constituted the control group?



The treatment was the Salk vaccine
The treatment group was the group of children that received
the Salk vaccine
The control group was the group of children that received
the injection of salt water
Sec. 1.3
Confounding Variables

Confounding variables are variables that are
unaccounted for that may affect a study

Ex. ~ Consider an experiment in which a statistics teacher
seeks to determine whether students who study in study
groups earn higher grades than students who study
independently. Suppose that at the end of the semester, the
teacher found that students who studied in study groups
earned higher grades. Furthermore, suppose that
(unbeknownst to the teacher) those students all lived in a
dormitory where a curfew ensured that they got plenty of
sleep.

The new variable amount of sleep, would be considered a
confounding variable because it could have been the reason (or
part of the reason) that those students had higher grades, but
it was not initially accounted for.
Sec. 1.3
Assigning Treatment and Control Groups

Researchers generally employ two
strategies to prevent confounding in
treatment and control groups
 Assigning
the subjects to the treatment
and control groups at random
 Using sufficiently large groups so that they
are unlikely to differ in a significant way
(like all living in a dorm with a strict
curfew)
Sec. 1.3
The Placebo Effect


The placebo effect refers to the situation in which
patients improve simply because they believe they are
receiving a useful treatment
A placebo is something that looks or feels just like
the treatment being tested, but lacks the active
ingredients

Placebos are used to distinguish between results caused by
the placebo effect and results that are truly due to the
treatment

Researchers try to make sure that the participants do not know
whether they are part of the treatment group or the control
group and they give the control group the placebo and the
treatment group the real dose.
Sec. 1.3
Example 4

What was the placebo in the Salk polio
vaccine study? Why did researchers use a
placebo in this experiment?
 The
salt water injection was the placebo in this
study
 To differentiate between improvements from the
vaccine and improvements from the placebo effect
Sec. 1.3
Experimenter Effects

An experimenter effect is a type of confounding
that occurs when the experimenter somehow
influences the results of the study


Typically occurs through facial expression, tone of voice, or
attitude change
This can occur when the experimenter knows who
received the treatment and who didn’t

Ex. ~ Consider an experiment that is testing an anti-depressant
drug in which the experimenter will interview the patients to
find out if they are feeling better after a certain time frame

If the experimenter is conducting the interviews and they know
who received the drug and who received the placebo, then they may
be inclined to sway the participant’s moods using facial expressions,
a certain tone of voice, or different attitudes.
Sec. 1.3
Blinding

Blinding is the practice of keeping people in
the dark about who is in the treatment group
and who is in the control group
 Single-blind
experiment – participants do not
know what group they are in, but experimenters do
 Double-blind experiment – neither the
participants nor the experimenters know who
belongs to each group

The researchers that are conducting the study typically
hire experimenters in a double-blind experiment to keep
track of the two groups and make any necessary contact
with the participants
Sec. 1.3
Example 5

For each of the experiments described below, identify any
problems and explain how the problems could have been avoided

A new drug for attention deficit disorder (ADD) is supposed to
make affected children more polite. Randomly selected children
suffering from ADD are divided into treatment and control groups.
The experiment is single-blind. Experimenters evaluate how polite
the children are during one-on-one interviews

Since the experimenters know which children received the drug, they
may inadvertently speak differently or interpret the children’s
behaviors differently based on what they want to see or hear
(experimenters effect).


This experiment should have been double-blind
A chiropractor performs adjustments on 25 patients with back pain.
Afterward, 18 of the patients say they feel better. He concludes
that the adjustments are an effective treatment

The 25 patients are all being “treated,” but there is no control group to
compare the results to. These patients may be feeling better as a
result of the placebo effect.

The experiment should have had a control group; maybe an actor that
pretends to adjust the patient the same as the real chiropractor.
Sec. 1.3
Meta-Analysis

Meta-analysis is a type of study in
which researchers review many past
studies as a whole
 The
goal in this type of study is to find
trends in the group that may not have been
evident in each individual study

See case study on P.31 for an example of metaanalysis
Sec. 1.3
Summary





Two basic types of statistical studies: Observation &
Experiment
Sometimes observational studies need to be
retrospective or prospective
Experiments need controls (treatment groups and
control groups) to avoid suffering from confounding
Furthermore, some experiments require blinding
(either single or double) to avoid suffering from
confounding (mainly the placebo effect and the
experimenter effect)
A third type of study is a meta-analysis which is
beneficial when there are previous studies that have
already been done