The Design of a Statistical Study
Notes
The basic structure of a statistical study has these guidelines:
1. Identify the variable(s) of interest and the population of interest.
2. Detail how to collect data.
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If a sample is used, make sure it represents the population.
3. Collect data.
4. Create descriptive statistics from the data.
5. Make decisions from the data using inferential statistics.
6. Identify any possible obstacles or errors.
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Data Collection
Notes
Types of Data Collection:
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Do an Observational Study
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Perform an Experiment
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Use a Simulation
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Use a Survey
Example: An observational study on mouthing behavior of nonfood
objects of children up to three years old is a suitable method to
collect data. Why are each of the other three methods not suitable
for this study?
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Data Collection - More Examples
Notes
Which method of data collection would you use to collect data for
each study? Explain your reasoning.
Example 1 A study of the relationship of eating salty foods and high
blood pressure.
Example 2 A study of opinions of independent films.
Example 3 A study of the effect on athletes in high elevation cities.
Example 4 A study on the effect of car crashes on humans.
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Obstacles in Experimental Design
Notes
A well designed experiment has three basic parts: control,
randomization, and replication.
Control
Influential factors on a variable must be controlled, but the variaty of
factors may make this difficult.
A confounding variable occurs when an experimentor cannot
distinguish between the effects of different factors on a variable.
Example: A famous Australian study showed that as ice cream sales
went up, so did the number of drownings. Higher ice cream sales and
higher numbers of drowning occur at the same time of year during
the summer. The time of year in this case is the confounding variable.
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Obstacles in Experiment Design
Notes
Two other issues with control:
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Placebo effect: Subjects are given no medicated treatment, but
reacts favorably.
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Hawthorne effect: Subjects are aware of treatment, so they
change their behavior.
To minimize these effects, an experimentor might use blinding or
double-blinding.
Example The ”Pepsi challenge” blindfolds a subject and asks him or
her to drink from two different sodas. Then he or she is asked to rate
their preference of each soda. Is there still a way to create a bias in
this experiment?
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Obstacles in Experiment Design
Notes
Randomization
Randomly assigning subjects to different treatment groups minimizes
experimental bias
Replication
Repetition of an experiment under the same or similar conditions give
more validity to an experiment.
Other elements of an experimental design
Sample size - The number of subjects. The larger the sample size,
the more likely it is to detect and validate results. Blocks,
Matched-Pairs - Assignment of subjects amongst others with similar
characteristics (e.g. age, location, a physical trait).
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Sampling Techniques
Notes
To count or measure an entire population, a census is used.
It is more common to sample a part of the population.
Sampling techniques:
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Simple random sample - Each member of a population has an
equal chance to be randomly sampled.
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Stratified sample - Each member of a subset, sharing a similar
trait, is likely to be sampled.
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Cluster sample - Each subset is likely to be sampled.
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Systematic sample - Order each member of the population and
select on member on a regular interval.
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Convenience sample - Sample members that are only available at
a particular time or location.
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Sampling Techniques
Notes
You design a study of student opinions on stem cell research. Identify
the sampling technique you use if you select the samples listed:
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You select a class at random and question each student in the
class.
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You assign each student a number and, after choosing a starting
number, you question every 25th student.
How is each sample selected and with what sampling technique?
Discuss, if any, potential sources of bias. Explain why it may trouble
inferential statistics.
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Assignment
Notes
For May 18 - 2014
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§1.3 #37, 40, 41, 42, 45 (due 23 May)
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Study the Chapter One Quiz on page 31.
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Read §2.1 by May 28
Suggested Exercises: 1, 2, 3, 4, 13, 16, 17, 21, 24, 25
You should be able to:
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Understand four methods of collecting data.
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Understand the design of an experiment.
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Understand random samples are created from many techniques.
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