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Planning a StudyDeciding what

and how to measure

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Vocabulary

Me asuring What?U

nitsExperimental UnitsSubjects

Participants

V arious V ariabl e s

Explanatory(independent) variableResponse (dependent)variableConfounding variableLurking variable

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Experiment

Subjecting the sample to acontrolled treatment whereone variable is altered.

The objects on which thetreatment is imposed on arecalled experimental units(human subjects ).

Explanatory variables arecalled factors and specificvalues of the explanatoryvariable are levels .

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Randomization--CrucialResearchers do experiments to reduce the likelihoodthat the results will be affected by confounding

variables and other sources of bias.

Randomize Type of Treatment

Randomize Order of Treatment

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Control GroupsControl group --receives standard treatment OR Placebo (sham) group--receives no treatment

Single-BlindDouble-Blind

T h e se control for UNKNOWN variability

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Quitting Smoking w/Nicotine PatchesRecruited 240 smokers (volunteers) at Mayo Clinic

from 3 large citiesRandomly assigned 22-mg nicotine

patch or placebo patch for 8 weeks.All attended counseling before, during, and after.Double-blind (neither volunteers nor nurses taking

measurements knew type of patch)After 8-wk (1 yr), 46% (27.5%) of nicotine patch

group quit smoking and 20% (14.2%) of placebo

group quit.

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Observational Study

Observing the behaviors of asample from a

population.

The observer does notimpose activetreatments onunits/subjects.

Or using previouslycollected data to dostatistical analysis.

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Census--Observational Study

The systematicalcollection of dataon the entire

population.

When the populationis large, it will

become timeconsuming andexpensive.

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Sample Survey--Observational Study

A portion of the

population is askeda question and thestudy is done basedon their voluntaryanswers.

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03-08-93 N ewsweek announced A Really Bad Hair Day:Researchers link baldness and heart attacks. The article

reported that men with typical male pattern baldnessare anywhere from 30 to 300 percent morelikely to suffer a heart attack than men with little or nohair loss at all.

The report was based on an observational study conducted by researchers at Boston Univ. School of Medicine. Theycompared 665 men who had been admitted to the hospitalwith their 1 st heart attack to 772 men in the same agegroup (21- to 54-years old) who had been admitted to thesame hospital for other reasons.

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Case Control Studies--

Observational StudyCases who have a specific

attribute/condition are

compared to Controls whodont.L EfficiencyL Reduces potential confounding

variablesL Retrospective vs. Prospective

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Characteristics of a well-designed

and well-conducted survey

L Train

ed int

ervi

ewers must b

econsist

entw ith asking n e utral, non-l e ading

qu e stions.L An unbias e d sampling should r e pr e se nt

th e population of int e r e st.

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PopulationsPopulations

Random SelectionsRandom Selections

SamplesSamples

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Simple Random SamplingFrom the entire population every

possible grouping of specified size hassame chance of being selected.

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Stratified RS vs Cluster S1st divide population

into groups (strata),

then take a SimpleRandom Sample fromeach strata

1st divide populationinto groups (cluster),

then randomly selectsome clusters andsample everyone inthat cluster

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Systematic Sampling & Random

Digit DialingFrom a list, divide into

consecutive segments

(every 50 names),randomly choosestarting point (21stentry), then sample atthat same point ineach segment (21, 71,121, 171, )

Sample thatapproximates a

SRS of allhouseholds in USthat havetelephones with aspecific exchange

(210-695- )

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Multi-Stage Samplingsurvey designers might stratify population by

region of country, then stratify by urban,suburban, or rural, then choose a randomsample of communities within those strata.They would continue to divide communities

into city blocks (fixed areas) as clusters, andsample from the selected clusters.

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Se lf-S e le ct e d Sampl e --radio station call-in

Conv e ni e nc e Sampl e --surveying folks in a

mall who appear willing to talk to you

Quicki e Polls-- hastily designed, poorly

pre-tested, one night survey sample for evening news show

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Sources of bias in surveysIf a selection process

consistently obtainsvalues too high or too low, then BI ASexists.

L Selection BiasL Non-response BiasL Response Bias

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Survey QuestionsUnnecessary complexityto questionMisleading questionOrdering of questionsEnsuring confidentialityAnonymous survey

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Gath

ering Data

Exp e rim e ntalDe sign

Obs e rvationalStudy