Table 3 Comparing the SWD to the parallel group CRT: aspects of study execution

Informed consent

~

(3a)*

May be difficult to obtain from subjects at the start of the study [13], and both SWDs and parallel group CRTs need modified informed consent procedures [57]

In cross-sectional SWDs the informed consent is in essence similar to that of a parallel group CRT. In cohort SWDs participants will have to understand that the moment of receiving the new intervention is being randomized

Study participation

-

(3b)*

An SWD may have increased risk of drop-outs and drop-ins (contamination) [6, 13, 18, 21, 22, 54]

† Inclusion rate

-

(3c)*

An SWD suffers relatively more from low inclusion rates because adding a cluster or extending the steps during the trial disrupts the symmetry of the design

† Study duration

~

(3d)

The possible longer study duration of SWDs might require interim analyses to avoid long exposure of clusters of participants to suboptimal care when the new intervention would be clearly inferior/superior to usual care. The statistical analysis aspects of interim analyses in an SWD are, however, still unclear

† Number of measurements

-

(3e)

If collecting data on health outcomes or costs is expensive, it may not be feasible to collect health economic evidence at each time point (step) in a cohort (longitudinal) SWD. This is particularly relevant if the number of steps (and hence number of measurements per participant), would be large. Even though a similar parallel group CRT would require more participants it might require fewer measurements in total [5, 34–36], and therefore could be more feasible

-

(3f)

Repeated measurements within the SWD may lead to a higher burden on everyone involved in the study. In the cross-sectional setting, this will not be a problem for individual participants, but may still be for research personnel [13, 16, 18, 19, 22, 35]

Blinding

-

(3g)

Blinding of participants and care providers is often impossible within SWD, however this also holds for the parallel group CRT. Hence, blinding of assessors of the outcomes is advised [6, 11–13, 19]

Improving intervention

~

(3h)

Within the SWD it is possible to improve the intervention during the study, though it is questionable whether it is desirable to do so [12, 18, 22, 56]