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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Hopkins WG: A Decision Tree for Controlled Trials, Sportscience 9, 2005.Hopkins WG, Marshall SW, Batterham AM, Hanin J: unpublished stats guidelines manuscript.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
Consider using several methods to gather information, then demonstrate congruence of data and concepts (triangulation).
Quantitative Clinical Case
Quantitative NonClinical Case
Causation in Observational Studies
That is, X is related to Y, but changing X may not change Y.
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
We are interested in X causing Y, so somehow we have to work out how much of the effect is not due to confounders.
But holding covariates constant is usually problematic.
We also measure the contribution of a potential mechanism by including it as a covariate in the linear analysis model.
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.
The control group solves one major problem but creates others.
The effect of a difference between groups in administration, compliance or imbalance can be attributed to a mediator with different mean values in the groups.
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
For a mechanisms analysis, create a similar figure with the change score of the potential mechanism as the covariate.
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
And the difference between the unadjusted and adjusted effects on the dependent (not shown) is the contribution of the covariate.
Measure all potentially important confounders and modifiers (subject characteristics and differences in conditions or protocols that could affect the effect).
You almost invariably analyze with some kind of linear model.
Case Series
A nonclinical case series is used:
e.g., not good for addressing movement patterns as a risk factor for ACL injury, but excellent for its genetic risk factors.
To avoid selection bias with choice of controls…
CaseCrossover
If blinding is not possible, try to include a mechanism variable not affected by expectation (placebo and nocebo) effects.
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…
Prepost Parallel Groups
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+
Will the intervention wash out in
an acceptable time for a crossover?
NO
YES
Prepostparallel groupsn=20+
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
Choose the most costeffective criterion.
Sample size is similar to that for validity studies, but no. of trials?
Perform extensive pilot work with experts and subjects to develop or modify wording in an exploratory factor analysis.
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Reference: Hopkins WG. Research designs: choosing and finetuning a design for your study. Sportscience 12, 1221, 2008