Stepped wedge designs

Issues and suggestions

JN. WM CLAHRC meeting June 2015

Issues to consider

• Precision of different designs• Bias – time trend• Study duration and dropout• Study cost• Ethics• Logistics

Time trend

• Any time trend confounds the intervention effect in SW designs because most control outcomes are measured early and most intervention outcomes late

• Needs to be allowed for in the analysis• But how? What model? Is it linear? Constant across

clusters?

Why introduce this uncertainty unless its necessary?

Study duration

The Stepped Wedge Cluster Randomised Trial

The Stepped Wedge Cluster Randomised Trial

Study duration

• Why design the study to last Txn steps, rather than Tx1 or Tx2 as in classic cluster trials?

• It leads to delayed results, additional research costs, the risk of ‘drop-out’, the risk of unexpected evolution of interventions, problems with time trend, etc

Study duration

• Why design the study to last Txn steps, rather than Tx1 or Tx2 as in classic cluster trials?

• It leads to delayed results, additional research costs, the risk of ‘drop-out’, the risk of unexpected evolution of interventions, problems with time trend, etc

There may be additional issues with the number of measurements and the precision of estimates which mean repeated x-sectional observations within clusters do need to be made

Study costs

• Important question is the ‘excess treatment costs’ ie the costs of introducing cluster interventions

• In the ‘classic’ SW the intervention is introduced in all clusters.

• In a classic parallel group cluster trial in just ½. But 1:many designs are common, easy to design, and potentially much cheaper

Ethics

• A question has been raised about the ethics of designing a study to deliver an unproven, experimental intervention to all the clusters, ie to everyone.

Could we introduce sequential SW designs, in which the effect would be tested at each step, and early stopping for effectiveness or futility would be allowed?

The design of the SW, with its sequential roll-out, seems ideally suited to this.

Logistics

• Some interventions can be introduced at the same time in many clusters

• But for many policy, public health, population or area wide interventions this may not be possible (practically or politically)

• If the intervention has to be rolled out in waves , randomise the roll out

• If randomisation isn’t possible, can still evaluate using a wave design

Non-randomised wave studies:The evaluation of NHS Direct

“NHS Direct was introduced in four discrete waves (March 1998, March 1999, December 1999 and November 2000) and we assumed that all NHS Direct sites in each wave started at the same time. We allocated each cooperative, ambulance service and emergency department to a wave. Services in Scotland, which had no helpline during the period examined, were allocated to ‘wave five’. “

Logistics

• The SW design seems to require that outcomes are measured in all clusters during the whole duration of the trial, typically being repeated at each step

• Is this expensive, burdensome, necessary?

• Why not roll out data collection as well as implementation?

Staircase designs

summary

• Precision aside, it comes down to the logistics• If the intervention can be introduced ‘all at once’ use

a classic parallel group cluster trial• If the intervention has to be rolled out in waves:– Randomise the order if possible, and consider

• Sequential analysis• SW designs or Staircase measurement

– If you can’t randomise, still evaluate using the methods of SW trials

Thank you