SCI1020: Introduction to Statistical Reasoning
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SAMPLING and EXPERIMENTAL DESIGNS
- How data are collected Student's Name:
Tutorial Day/Time:
PRELIMINARY READING: D S Moore et al, “Basic Practice of Statistics”, Chs. 8& 9.
On completion of this workshop you should be able to:
1. Select an appropriate design for a given example of a study.
2. Distinguish between observational and experimental study designs and identify features of the
design such as variables, blocking, placebo, and blinding.
3. Identify the principles of statistical design of experiments: control, randomisation and replication
4. Identify these features in a randomized comparative experiment by constructing a schematic chart
for the design,
5. Identify a confounding variable and how it is incorporated into a randomized block experimental
design,
6. Identify a matched pair experimental design.
7. Distinguish between different sampling methods and identify the most appropriate sampling
method for the given data.
Part A: SAMPLING
Q.1 State what you understand by the following terms for bias in surveys. Read Moore,
Chapter 8.6 on ‘Cautions about sample surveys’ but use your own words here.
Bias
Description
Undercoverage bias
Response bias
Non-response bias
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Q.2 From the scenarios given in Qs 8.1 to 8.4 from Moore text, state what type of sampling
design is used in each and the possible problem with it:
Method
Population
Sample
Sampling design
type
Main possible
bias?
Q8.1
Q8.2
Q8.3
Q8.4
Part B: EXPERIMENTAL DESIGN
SCHEMATIC OUTLINE DIAGRAMS will be used to describe studies.
Schematic outlines or flow-charts, are a useful way to illustrate the steps required in a
particular study. Outlines of observational, case-control study, randomized experiment
(simple and block designs) are shown on pp.6-7 of this handout.
For specific examples of outlines for experiments, see Moore:
• Randomized comparative experiment Figs 9.3 & 9.4
• Blocked experiment
Fig. 9.5
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Q.3 .Complete the following table as a class discussion.
Study
Circle what Type?
1.. Can antidepressant pills or
stress management reduce the
number and severity of chronic
tension-type headaches?
2. Measurements of the
incidence of a disease in two
different locations to
determine any differences
3. Does smoking increase the
risk of developing bowel
cancer?
Random Experiment
Explanatory
variable
Response
variable
Observational study
Case-control study
Random Experiment
Observational study
Case-control study
Random Experiment
Observational study
Case-control study
Q.4 Do Q9.39 from Moore text. This randomized experiment involves more than 1 factor. It
is an example of a 2-factorial randomised experiment.
Explanatory variables and their levels
1.
Response variable =
(Be very specific!)
2.
Total sample size required =
Outline:
NOTE: The main advantages of factorial experiments are…
1.
2.
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Q.5 Do Q9.46 from Moore, 9th ed.
On September 1, 2018, The Columbus Dispatch reported on a study about political polarization
and social media. In this study, 901 Democrats and 751 Republicans were recruited. The
Democrats were randomly divided into two groups. All were asked to follow an automated
Twitter account (Twitter bot) each day for one month. One group received tweets with a liberal
point-of-view, and the other tweets with a conservative point-of-view. Likewise, Republicans
were randomly divided into two groups and received the same two treatments (liberal or
conservative tweets). All subjects were given a test, both before and after the experiment, that
scored them on a liberal/conservative scale. Changes in scores were the response variable.
a) type of experimental design =
b) Outline
Q.6 : Do Q9.43 from Moore text. Liquid water enhancer
Do this as class discussion as a specific Matched pairs design.
Explanatory variable
Response variable:
Specify what you will measure
and how.
What makes this a “matched
pair” design? Consider the
advantages of a repeated
measure design!
Is blinding a possible feature?
If so, how?
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IF randomisation is used:
What is randomised?
What replication is used?
What is the “control”?
Draw a schematic diagram OUTLINE for your proposed Liquid water enhancer study,
remembering to show the SAME matched pair (or repeated measure) as discussed.
(Check the basic types of outlines possible at the end of this handout!)
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APPENDIX
Generic layout of Outlines for different STUDY DESIGNS
Note that it is important to clearly identify if you are randomly selecting participants, or if
you are sorting people into groups (blocking) at each stage in the process. Also state clearly
what property is to be measured as the response variable.
RANDOMIZED COMPARATIVE EXPERIMENT
(eg Moore: Fig. 9.3 & 9.4 , pp.237-238) (7th ed, pp234-235)
SAMPLE
RANDOM ALLOCATION of units,
approx. same numbers in each
Treatment Group 1
Treatment Group 3
(control)
Treatment Group 2
Measure response variable in each group and compare
(the various levels of the explanatory variable are in treatment groups)
BLOCKED RANDOMIZED EXPERIMENT
(eg Moore: Fig. 9.5 , p.245) (7th ed, p243)
SAMPLE
SORTING according to the CONFOUNDING Variable
BLOCK 1
RANDOM ALLOCATION
T1
T2
T3
BLOCK 2
BLOCK 3
RANDOM ALLOCATION
T1
T2
T3
RANDOM ALLOCATION
T1
T2
T3
Specify what is measured in response variable in each group
and compare within the Block and also between the Blocks.
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MATCHED PAIRS EXPERIMENT are forms of the above where
(i) pairs of matched individuals are BLOCKS (eg age/gender/ethnicity/location)
OR
(ii) Repeated measure: Blocks are individuals. The treatment group includes both
treatments, given in random order.
SAMPLE
RANDOM ALLOCATION of units,
approx. same numbers in each
Treatment Group 1
Treatment Group 2
Treatment Group 2
Treatment Group 1
Measure the difference in the response variable between the treatments in each stream and comp the
different orders
OBSERVATIONAL STUDY
SAMPLE
SORTING
according to assessment of Explanatory Variable
“naturally” present in the subjects
Result A
Result B
Result C
Assessment of response variable in each Result and compare
CASE-CONTROL STUDY
- a specific type of observational study with some form of comparative
control
Sample of cases
(response variable present)
Sample of controls
(known to have zero response)
Observe in each the extent of the explanatory variable
…………………………………………………….
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