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GATHERING DATA
Chapter 4
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4.3 What are Good and Poor Ways to Experiment?
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Elements of an Experiment
¨ Experimental units: Subjects
¨ Treatment: Conditions imposed on subjects
¨ Explanatory variable: Defines groups and treatments
¨ Response variable: Outcome
www.clinplus.com
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Experiments
¨ Impose treatments on subjects to observe responses
¨ Goal: compare effects of treatments on response
¨ Randomized experiments – subjects randomly assigned to treatments
www.privatemdlabs.com
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Placebo effect
Placebo – fake treatment; sugar pill
Placebo effect –improving not from real treatment but from belief that he or she should improve
www.scientificamerican.com
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3 Components of a Good Experiment
¨ Control or Comparison Group ¨ Randomization ¨ Replication
pages.cs.wisc.edu
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Principle 1: Control or Comparison Group
¨ Helps analyze effectiveness of primary treatment
¨ Placebo removes lurking variables
¨ Control group gets placebo ¤ Clinical trials may
compare new treatment with existing
www.redicecreations.com
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¨ Experiments should compare treatments rather than effect of single treatment
¨ Example: 400 volunteers asked to quit smoking with some taking and some not taking antidepressant
Control or Comparison Group
www.gpaulmckinneyjr.com
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Principle 2: Randomization
1. Eliminates bias from researcher assigning subjects
2. Balances groups on known and lurking variables
colorectal.surgery.ucsf.edu
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Principle 3: Replication
1. Reduces difference due to ordinary variation or chance
2. Increases chance that results show true difference
www.printercopierblog.com
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Blinding the Experiment
¨ Blind – subjects unaware of which treatment used
¨ Double-Blind Experiment - Neither subjects nor investigators know which treatment ¤ Controls bias from
respondent and experimenter
gridskipper.com
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Statistically Significant Difference – Observed difference is larger than expected from chance
Statistical Significance
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Generalizing Results
¨ Goal of Experimentation – Analyze association between treatment and response for entire population
¨ Generalize only to population represented by study
¨ Page 180 #34, 40
static.howstuffworks.com
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4.4 Other Ways to Conduct Experimental and Observational Studies
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Sample Surveys: Random Sampling Designs
¨ Alternative to experiments 1. Simple Random
Sampling 2. Cluster Random
Sampling 3. Stratified
Random Sampling
codetechnology.files.wordpress.com
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Cluster Random Sample
1. Divide population into large number of clusters, such as city blocks
2. Select simple random sample of clusters
3. Use all subjects in clusters as sample
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Advantages ¤ Sampling
frame unavailable
¤ Cost Disadvantage
¤ Need larger sample size for same reliability
Cluster Random Sample
reactorfire.files.wordpress.com
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Stratified Random Sample
1. Divide the population into groups, strata
2. Select SRS from each strata
3. Combine samples from each for total sample
www.nedarc.org
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Advantage ¨ Ensures stratum
representation Disadvantage ¨ Need sampling
frame and to which stratum each subject belongs
Stratified Random Sample www.smh.com.au
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Comparing Random Sampling Methods
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Types of Observational Studies
1. Sample Survey: current
2. Retrospective Study: past
3. Prospective Study: future
¨ Cause not proven, but studies can support beliefs
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Retrospective Case-Control Study
www.lisisoft.com
Studying sunlight exposure and multiple sclerosis connection…
Retrospective
Cases – have MS Controls or don’t
Explanatory variable – low sun or not
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Prospective Case-Control Study
www.dubaiinternetmarketing.com
Studying effects of
vegetarian diet on heart
disease…
Prospective
Cases – have heart disease Controls or don’t
Explanatory variable – vegetarian or not
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Multifactor Experiments
¨ Single experiment analyzes two or more factors
¨ Learn more since combinations may affect response
biobreak.files.wordpress.com
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Matched Pairs Design
¨ Subjects are somehow matched ¤ Husband/wife, two plots in
same field, etc. ¤ Same individual – crossover
design ¨ Randomly assign or randomize
order of treatments ¨ Reduces effects of lurking
variables
www.marshallhammondwedding.info
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Randomized Block Design
¨ Block – subjects with common characteristics ¨ Randomized Block Design, RBD – within each block,
randomly assign to treatments