Post on 20-Jun-2020
4.2 Experiments
Learning Objectives
After this section, you should be able to:
• DISTINGUISH between an observational study and an experiment.
• EXPLAIN the concept of confounding.
• IDENTIFY the experimental units, explanatory and response variables, and treatments in an
experiment.
• EXPLAIN the purpose of comparison, random assignment, control, and replication in an
experiment.
• DESCRIBE a completely randomized design for an experiment.
• DESCRIBE the placebo effect and the purpose of blinding in an experiment.
• INTERPRET the meaning of statistically significant in the context of an experiment.
• EXPLAIN the purpose of blocking in an experiment. DESCRIBE a randomized block design or
a matched pairs design for an experiment.
Observational Study vs. Experiment
An observational study observes individuals and measures
variables of interest but does not attempt to influence the
responses.
An experiment deliberately imposes some treatment on
individuals to measure their responses.
When our goal is to understand cause and effect, experiments are the only
source of fully convincing data.
The distinction between observational study and experiment is one of the most
important in statistics.
Observational Study vs. Experiment
A response variable measures the outcome of a study.
An explanatory variable may help explain or predict changes in
a response variable.
Check Your Understanding
Does reducing screen brightness increase battery life in laptop
computers? To find out researchers obtained 30 new laptops
of the same brand. They chose 15 of the computers at random
and adjusted their screens to the brightest setting. The other
15 laptops were left at the default setting – moderate
brightness. Researchers then measured how long the
machine’s battery lasted.
Was this an observational study or an experiment? Justify you answer.
Experiment, because the treatment (brightness of screen) was imposed
on laptops.
Check Your Understanding
Does eating dinner with their families improve students’ academic performance?
According to an ABC News article, “Teenagers who eat with their families at least five
times a week are more likely to get better grades in school.” This finding was based on
a sample survey conducted by researchers at Columbia University.
1. Was this an observational study or an experiment? Justify your answer.
Observational study, because students were not assigned to eat particular number of meals
with their family per week.
2. What are explanatory and response variables?
Explanatory: Number of meals per week eaten with their family
Response: GPA
Check Your Understanding
Does eating dinner with their families improve students’ academic performance?
According to an ABC News article, “Teenagers who eat with their families at least five
times a week are more likely to get better grades in school.” This finding was based on
a sample survey conducted by researchers at Columbia University.
Explain clearly why such a study cannot establish a cause-and-effect relationship.
Suggest a variable that may be confounded with whether families eat dinner together.
There are probably other variables that influence the response variable. For example,
students who have part time jobs may not be able to eat many meals with their
families and may not have much time to study, leading to lower grades.
Observational Study vs. Experiment
Observational studies of the effect of an explanatory variable on a response
variable often fail because of confounding between the explanatory variable
and one or more other variables.
Well-designed experiments take steps to prevent confounding.
Confounding occurs when two variables are associated in
such a way that their effects on a response variable cannot
be distinguished from each other.
The Language of Experiments
An experiment is a statistical study in which we actually do something (a
treatment) to people, animals, or objects (the experimental units) to
observe the response. Here is the basic vocabulary of experiments.
A specific condition applied to the individuals in an experiment
is called a treatment. If an experiment has several explanatory
variables, a treatment is a combination of specific values of
these variables.
The experimental units are the smallest collection of
individuals to which treatments are applied. When the units are
human beings, they often are called subjects.
How to Experiment Badly
Many laboratory experiments use a design like the one in the online
SAT course example:
Experimental
UnitsTreatment
Measure
Response
In the lab environment, simple designs often work well.
Field experiments and experiments with animals or people deal with more
variable conditions.
Outside the lab, badly designed experiments often yield worthless results
because of confounding.
How to Experiment Well
The remedy for confounding is to perform a comparative experiment in which some
units receive one treatment and similar units receive another. Most well designed
experiments compare two or more treatments.
Comparison alone isn’t enough, if the treatments are given to groups that differ
greatly, bias will result.
The solution to the problem of bias is random assignment.
In an experiment, random assignment means that
experimental units are assigned to treatments using a
chance process.
Principles of Experimental Design
Principles of Experimental Design
The basic principles for designing experiments are as follows:
1. Comparison. Use a design that compares two or more treatments.
2. Random assignment. Use chance to assign experimental units to
treatments. Doing so helps create roughly equivalent groups of
experimental units by balancing the effects of other variables among
the treatment groups.
3. Control. Keep other variables that might affect the response the same
for all groups.
4. Replication. Use enough experimental units in each group so that any
differences in the effects of the treatments can be distinguished from
chance differences between the groups.
Completely Randomized Design
In a completely randomized design, the treatments are assigned to all the
experimental units completely by chance.
Some experiments may include a control group that receives an inactive
treatment or an existing baseline treatment.
Experimental
UnitsRandom
Assignment
Group
1
Group
2
Treatment
1
Treatment
2
Compare
Results
Check Your Understanding
Music students often don’t evaluate their own performances accurately. Can small group discussions help? The subjects were 29 students preparing for the end-of-semester performance that is an important part of their grade. The 15 students in one group each videotape a practice performance, evaluated it themselves, and then discussed the tape with a small group of other students. The remaining 14 students watched and evaluated their tapes alone. At the end of the semester, the discussion-group students evaluated their final performance more accurately.
1. Describe a completely randomized design for this experiment. Write a few sentences describing how you would implement your design.
Randomly assign 29 students to two treatments: evaluating the performance in small groups or evaluating the performance alone. The response variable will be the accuracy of their final performance evaluations. To implement this design, use 29 equally sized slips of paper. Label 15 of them “small group” and 14 of them “alone.” Then shuffle the paper and hand them out at random to the 29 students to assign them to a treatment.
Check Your Understanding
Music students often don’t evaluate their own performances accurately. Can small group
discussions help? The subjects were 29 students preparing for the end-of-semester
performance that is an important part of their grade. The 15 students in one group each
videotape a practice performance, evaluated it themselves, and then discussed the tape
with a small group of other students. The remaining 14 students watched and evaluated
their tapes alone. At the end of the semester, the discussion-group students evaluated
their final performance more accurately.
What is the purpose of the control group in this experiment?
The purpose of the control group is to provide a baseline for comparison.
Without a group to compare to, it is impossible to determine if the small group of
treatment is more effective.
Experiments: What Can Go Wrong ?
The logic of a randomized comparative experiment depends on our ability to treat
all the subjects the same in every way except for the actual treatments being
compared.
Good experiments, therefore, require careful attention to details to ensure that all
subjects really are treated identically.
The response to a dummy treatment is called the placebo effect.
In a double-blind experiment, neither the subjects nor those who interact
with them and measure the response variable know which treatment a
subject received.
Inference of Experiments
In an experiment, researchers usually hope to see a difference in the responses so
large that it is unlikely to happen just because of chance variation.
We can use the laws of probability, which describe chance behavior, to learn
whether the treatment effects are larger than we would expect to see if only chance
were operating.
If they are, we call them statistically significant.
An observed effect so large that it would rarely occur by chance is called
statistically significant.
A statistically significant association in data from a well-designed experiment
does imply causation.
Blocking
When a population consists of groups of individuals that are “similar within
but different between,” a stratified random sample gives a better estimate
than a simple random sample. This same logic applies in experiments.
Blocking
A block is a group of experimental units that are known before
the experiment to be similar in some way that is expected to
affect the response to the treatments.
In a randomized block design, the random assignment of
experimental units to treatments is carried out separately
within each block.
Matched Pairs Design
A common type of randomized block design for comparing two
treatments is a matched pairs design. The idea is to create blocks
by matching pairs of similar experimental units.
A matched pairs design is a randomized blocked experiment in which each
block consists of a matching pair of similar experimental units.
Chance is used to determine which unit in each pair gets each treatment.
Sometimes, a “pair” in a matched-pairs design consists of a single unit that
receives both treatments. Since the order of the treatments can influence the
response, chance is used to determine with treatment is applied first for each
unit.