The dog-leg: an alternative to a cross-over design for pragmatic clinical trials in relatively stable populations.
BACKGROUND
A cross-over trial design is more powerful than a parallel groups design, but requires that treatment effects do not carry over from one period of the trial to the next. We focus here on interventions in chronic disease populations where the control is routine care: in such cases we cannot assume the intervention effect is easily washed out in crossing over from the experimental intervention back to the control.


METHODS
We introduce an alternative trial design for these situations, and investigate its performance. One group is assessed before and after the experimental intervention, whereas two other groups provide respective, independent treatment comparisons in each period. We call this a dog-leg design because of the pattern of assessments in the three groups. The dog-leg design is reminiscent of a stepped wedge design, but with a reduced schedule of assessments and with the notable difference that not all groups receive the intervention.


RESULTS
If the correlation between baseline and follow-up is <0.72, the dog-leg design is more efficient than a parallel groups design with a baseline assessment. The dog-leg design also requires fewer assessments in total than a parallel groups design where participants are only assessed once, at follow-up.


CONCLUSIONS
The dog-leg design is simple, and has some attractive properties. Though there is a risk of differential attrition in the three arms, the design's good performance relative to alternatives makes it a useful addition to the methodologist's toolkit.
Centre for Primary Care & Public Health, Queen Mary University of London, London, UK r.l.hooper@qmul.ac.uk.
Richard
R
Hooper
Centre for Primary Care & Public Health, Queen Mary University of London, London, UK.
Liam
L
Bourke
D002908:Chronic Disease / Q000628:therapy*; D018592:Cross-Over Studies; D006801:Humans; D016032:Randomized Controlled Trials as Topic / Q000379:methods*; D012107:Research Design; D018401:Sample Size
clinical trial design; cross-over trials; dog-leg; stepped wedge