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Solving Statistics: Stepped-wedge randomized controlled trial

Stepped-wedge randomized controlled trial


The performance of activities of daily living (ADL) at home is important for the recovery of older individuals after hip fracture. However, 20-90% of these individuals lose ADL function and never fully recover. Although exercise interventions have been proven to improve physical function, especially elderly do not seem to benefit from these interventions. In this prospective, stepped-wedge randomized controlled trial, care as usual [CaU] is compared to 1) occupational therapy (OT) with coaching based on cognitive behavioural treatment (CBT) [OTc], and 2) OT-CBT with sensor monitoring embedded [OTcsm]. More specifically, during 12 months, six nursing homes will start with providing CaU, then cross over to provide OTc and finally cross over to provide OTcsm. The timing of crossing over is randomized: two nursing homes will cross over for the first time after two months, two after four months and the last two after six months. OTc will always be provided for 4 months, CaU for two, four and siex months respectively, and OTcsm for six, four and two months. The primary outcome measure, perceived daily functioning, is measured 6 months after start of rehabilitation and compared to baseline functioning.1


Why would a stepped-wedge design be chosen over an ordinary randomized controlled trial?


Randomized controlled trials (RCTs) are considered to be the most reliable form of scientific evidence. Ideally, each individual participating in the trial is randomized to receive one of the treatments. However, especially in geriatric research in nursing homes, individual randomization is difficult and may introduce bias. Therefore, (wards of) nursing homes are generally randomized to treat all their inhabitants with one of the treatments. This is called cluster randomization. In a stepped-wedge RCT, the new treatment is rolled out sequentially over multiple time points, and usually the crossover from one treatment to the other is in one direction (from care as usual to new). Of note, in both a stepped-wedge RCT and a cluster RCT, participants are usually not blinded for the treatment, as is the case in the paper of Pol et al1, and therefore could be at risk of selection bias.

Now that we set the terminology, let’s look at differences between an ordinary (cluster) randomized controlled trial and a stepped-wedge randomized controlled trial. The first advantage of a stepped-wedge design is that it allows for phased introduction of the intervention. This is especially important when an intervention has been proven effective on subject level, but it is still under investigation whether implementation on population level is effective. By the end of the trial all clusters will apply the new treatment.

When the clusters are relatively heterogeneous (for example a nursing home in urban environment versus a nursing home in rural environment), the stepped-wedge design will have more statistical power than an ordinary cluster RCT. In one nursing home participants received either care as usual or new treatment (depending on the cross-over time), so each intervention group also has a control group in the same nursing home.

These are the two important differences between a cluster RCT and a stepped-wedge RCT. So, my answer to the question would be: I think for this specific trial, the stepped-wedge design was chosen over a cluster RCT because of the heterogeneity of the clusters. Nursing homes in the Northwest and Midwest part of the Netherlands would participate, covering both rural and urban parts of the Netherlands.

One last remark, on the statistics of stepped-wedge RCTs: in a stepped-wedge design, all clusters are exposed to the new treatment at end of the study so that its effect might be confounded with an underlying time trend. Therefore, the analysis of a stepped-wedge RCT should always consider confounding of time (which is not needed in a cluster RCT).

C Care as usual, OTc Occupational therapy with coaching, OTcsm Occupational therapy with coaching and sensor monitoring, NH= Cluster = Nursin ghome, Trial duration = 12 months (recruitment), 18 months (including exposure and measurements). Number of clusters = 6. Number of groups = 3. Number of clusters per group = 2 (corss over simultaneously). Pre-rollout period = 12 months. Rollout period = 8 months. Post-rollout period = 2 months. Step length (intervention 1-2) = 2 months. Number of participants per step =8.

Retrieved from: Pol MC, ter Roet G, van Hartingsveld M, Kröse B, de Rooij SE, Buurman BM. Effectiveness of sensor monitoring in an occupational therapy rehabilitation program for older individuals after hip fracture, the SO-HIP trial: study protocol of a three-arm stepped-wedge cluster randomized trial. BMC Health Serv Res. 2017 Jan; 17(1):3.

Statistical terminology highlighted

Randomized controlled trial (RCT): A clinical trial, in which participants are at random allocated to either receive a new treatment (intervention group) or no/standard treatment (control group). The random allocation minimises selection bias, and any differences between the two groups are due to change. An RCT is considered the gold standard to prove efficacy of a new treatment.

B.I. Lissenberg-Witte

Department of Epidemiology and Biostatistics, VU University Medical Center, Amsterdam, The Netherlands


  1. Pol MC, ter Roet G, van Hartingsveld M, Kröse B, de Rooij SE, Buurman BM. Effectiveness of sensor monitoring in an occupational therapy rehabilitation program for older individuals after hip fracture, the SO-HIP trial: study protocol of a three-arm stepped wedge cluster randomized trial. BMC Health Serv Res. 2017 Jan; 17(1): 3.

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