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RESEARCH DESIGNS:Choosing and finetuning a design for your study
Sources/AcknowledgmentsHopkins WG: Quantitative Research Design, Sportscience 4(1), 2000.Batterham AM, Hopkins WG: A Decision Tree for Controlled Trials, Sportscience 9, 2005.Hopkins WG, Marshall SW, Batterham AM, Hanin J: unpublished stats guidelines manuscript.
Will G Hopkins AUT University, Auckland, NZ
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Qualitative Cases
Quantitative Clinical Case
Quantitative NonClinical Case
Causation in Observational Studies
Effect of Confounders
Effect of Mediators
Covariates notconsidered
Z1
Effect of X on Yconfounded by Z1
Effect of X on Ymediated by Z2
X(activity)
X
X
X
Z1
(age)
Z2
(fitness)
Y(health)
Y
Y
Y
Effect of X on Ynotconfounded by Z1
Effect of X on Ynotmediated by Z2
Observed effect of X on Y
Effects involving Z and X or Z and Y
=
+
Effect of X on Y involving Z
Effect of X on Y not involving Z
Causation in Interventions
Mediators with Exptal Treatment T
Mediatorwith Control Treatment C
No Z can causeTreatment T
Z
Effect of T on Ymediated by Z1
T
C
T
Z2
Z2
Z1
Z
Y
Y
Y
Observed effect of T on Y
Effect of T on Ymediated by Z2
Effect of C on Ymediated by Z2
= effect due to mediator Z1
= unbiased effect of treatment T
= experimental treatment effect minus control treatment effect.
Difference in slopesimplies the pretest valueof the dependent mediatesindividual differences inthe effect of the treatment.
Postpre changein dependent
exptalgroup
mean experimental
effect adjustedto grand mean
Negative slope in controldue to regression to mean.
unadjustedeffect
mean control
E.g., treatment haszero net effect at this pretest value.
controlgroup
0
grandmean
Pretest value
Postpre changein dependent
mean experimental
exptalgroup
mean control
adjusted effect(effect not due to covariate)
0
unadjusted (full) effectof treatment
0
Postpre change in covariate
controlgroup
Case Series
Crosssectional Study
CaseControl Study
e.g., not good for addressing movement patterns as a risk factor for ACL injury, but excellent for its genetic risk factors.
CaseCrossover
Cohort Study
Prepost Single Group
washout
3 treatments need multiples of 6 subjects (6, 12, 18…); 4 need multiples of 4; 5 need multiples of 10; 6 need multiples of 6…
washout
Prepost Parallel Groups
Postonly Parallel Groups
Can you use a control group
or control treatment?
NO
YES
Is the measure
reliable
over the intervention period?
NO
YES
Prepostsingle group
Will the intervention wash out in
an acceptable time for a crossover?
n=10+
NO
YES
Is the measure
reliable
over
Postonly
washout+intervention
period?
parallel groups
n=300+
NO
YES
Are you limited
Pre

post
by subjects
parallel groups
or resources?
n=20+
NO
YES
Pre

post
crossover
Decision Treefor Choosing theBest Intervention
n=10+
Post

only
crossover
n=10+
Can you use a control group
or control treatment?
NO
YES
Prepost single group
n=10+
Is the measure
reliable
over the intervention period?
NO
YES
Postonlyparallel groupsn=300+
Will the intervention wash out in
an acceptable time for a crossover?
NO
YES
Prepostparallel groupsn=20+
Is the measure
reliable
over
washout+intervention
period?
NO
YES
Are you limited
by subjects
or resources?
NO
YES
Prepost
Postonly
crossover
crossover
n=10+
n=10+
Validity Study
Study of Diagnostic Accuracy
Reliability Study
Study of Factor Structure
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Reference: Hopkins WG. Research designs: choosing and finetuning a design for your study. Sportscience 12, 1221, 2008