Transcript Designing Samples

Silent Do Now (5 minutes)
*Before you begin, grab a new weekly sheet and take out your homework!
 An opinion poll calls 2000 randomly chosen residential
telephone numbers then asks to speak with an adult
member of the household. The interviewer asks, “how
many movies have you watched in a movie theatre
in the last 12 months?” In all, 1131 people respond.
 Identify a potential source of bias related to the question
being asked. Suggest a change that would fix the problem.
 Identify another potential source of bias in this survey that is
unrelated to the once you just mentioned. Suggest a
change that would help fix this problem.
Important Definitions
 Observational Study Vs. Experiment
 Tell it in your own words!
Important Definitions
 Experimental Units - Individuals on which the experiment
is being done
 Subjects – what we call experimental units that are
human
 Factors – explanatory variables in an experiment
 Levels – created through the joint effects of several
factors
 Response Variable – The output of your experiment
 Treatment – The experimental condition placed on the
experimental units
Important Definitions
 Control Group–
 The group that we compare to the treatment group.
 The group that receives either no treatment or a standard
treatment.
Important Definitions
 Placebo Effect – When patients respond favorably to a
dummy treatment (a placebo)
 The response can be due to trust in the doctor and
expectations of a cure or simply to the fact that conditions
often improve without treatment
Important Definitions
 There are two main classifications of people who can
affect the outcome of an experiment.
 Those who could influence the results (subjects, administrators of
treatments, etc)
 Those who evaluate the results
 Blind – Either the subjects or those who measure the
response variable (evaluators of the experiment) are
unaware of which group the subject has been assigned
to.
 Double Blind – Neither the subjects nor those who
measure the response variable (evaluators of the
experiment) know which treatment a subject received
Comparative Experiments
 In order to measure the causal effect of a
treatment, it is desirable to have more than
one group to compare results.
 This may mean we are comparing two or
more levels of a factor.
 It may also mean that we are comparing a
single level of a factor to a group which
does not receive the treatment.
Four Principles of Experimental Design
1.
Control
2.
Randomization
3.
Replication
4.
Blocking
The first three MUST be present in every experiment
in order to apply it to the general population, the
last is not always necessary.
Control
 Sources of variation in a variable can come from many
places
 We must identify any possible sources of variation besides
the treatment itself and then control these sources as best
we can.
For Example: If I am trying to see how rapidly bacteria will grow
on a petri dish in a laboratory, I must control the
environment of the laboratory to ensure that chemicals or
other substances don’t affect the bacteria.
Randomization
 Randomization is just as important in
designing an experiment as it is in selecting
a sample.
 Randomizing which individuals get which
treatments helps us to spread the variation
between individuals out across the groups.
 Randomizing is a way of maintaining control
over sources of variation.
Replication
There are two kinds of replications in
comparative experiments
 Applying the treatments to more than one
individual
 Replicating the entire study
 This allows us to generalize to the larger population
with more confidence.
Blocking
 Blocking, like stratification in sampling, allows
us to identify a variable we know will have
different results and thus control this variable
 To block we:
 Place all individuals in one group by the identified
variable
 We use randomization on the resulting groups to all
levels of the treatment.
For Example: If we block by gender, because we
believe that we will receive different results for
each gender, we essentially are running separate
experiments on each gender and analyzing the
results separately.
Types of Experimental Designs
 Completely Randomized Design – All experimental units
are allocated at random among all treatments.
 Example:
Types of Experimental Designs
 Randomized Block Design – Random assignment of units
to treatments is carried out separately within each block.
 Example:
Types of Experimental Designs
 Matched Pairs Design – Compares only two treatments.
 Subjects are matched in pairs
For Example: An experiment to compare two advertisements
for the same product might use pairs of subjects with the
same age, sex and income. The idea is that matched
subjects are more similar than unmatched subjects, so
comparing responses within a number of pairs is more
efficient than comparing responses of groups of randomly
assigned subjects.
Example of Matched Pairs Design
Repeated Measures Design
A repeated measures design is a form of
matched pair design where each
subject serves as their own control.
In other words, every subject will take
both the new medication and the old
medication and our response variable
will again be the difference in pain relief
for each subject.
How to Design an Experiment
1. Identify the important FEATURES of the experiment.
2. Draw a DIAGRAM for the randomized design.
3. Explain how the design utilizes the PRINCIPLES OF
EXPERIMENTAL DESIGN.
Example 1:
 A farm-product manufacturer wants to determine if the
yield of a crop is different when the soil is treated with three
different types of fertilizers. Fifteen similar plots of land are
planted with the same type of seed but are fertilized
differently. At the end of the growing season, the mean
yield from the sample plots is compared.
 Identification of Features:
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What are the experimental units?
What is the factor (explanatory variable)?
How many levels?
What is the response variable?
How many treatments?
 Diagram:
 Four Principles of Design:
Quick Check!
Example 2: Randomized Block Design
 A pharmaceutical company has developed a new
medicine that they believe will be more effective in
treating people who suffer from migraine headaches.
Researchers plan to enlist several people who suffer from
migraines in a test.
 Identification of Features:
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What are the experimental units?
What is the factor (explanatory variable)?
How many levels?
What is the response variable?
How many treatments?
 Diagram:
 Four Principles of Design:
Quick Check! (Pg. 374 #5.5 in
textbook!)
 Oops!!!
Help! Help! Help!
HOW DO WE STAY ETHICAL YET ELIMINATE
BIAS?
Informed Consent
A subject should have know that they
are a part of an experiment, and that
the possibility exists for them to be
assigned to one of several groups. This
way the subject can either choose to
participate or not. What they won’t be
told is which group they will be assigned.
Independent Practice
 Finish for homework (read carefully!!!)
 Pg. 5.52 & 5.53