Table 3 Reasons for having multicentre studies that do/do not stratify by centre (Question 2)
Unit has multicentre trials that do not stratify randomisation by centre? | Yes (N = 25) | No (N = 18) | ||
|---|---|---|---|---|
Reason(s) provided | ||||
Expected homogeneity of treatment effect across centres | 11 | 44% | 2 | 11% |
No interest in centre effect | 4 | 16% | 1 | 6% |
Lots of centres with few participants per centre | 19 | 76% | 1 | 6% |
Not convinced of appropriateness of either fixed or random effect models for centres in the trial | 1 | 4% | 0 | 0% |
Other reason provided | ||||
Aids in blinding if trial open label | 1 | 4% | 0 | 0% |
Balance against other important factors. Centre effect less important in drug trials compared to complex or surgical interventions | 1 | 4% | 0 | 0% |
Concern that, in an unblinded trial, stratifying by centre would make it easier to predict the treatment allocated to the next patient [12] 16:405). | 1 | 4% | 0 | 0% |
For practical reasons | 0 | 0% | 1 | 6% |
Intervention takes place out of hospital | 1 | 4% | 0 | 0% |
Large sample size with small/moderate number of centres. We expect balance to be achieved with simple randomisation | 1 | 4% | 0 | 0% |
Likely to stratify by geographical region if not by centre | 1 | 4% | 0 | 0% |
Randomisation system defaults to stratifying by centre but one example where minimised trial did not. Need to consider balance of resources and avoid confounding. There is a lot of academic debate. e.g. Torgerson. | 0 | 0% | 1 | 6% |
Sometimes stratify by region | 1 | 4% | 0 | 0% |
Stratified by treatment provider within centres and treatment providers unique within centre | 1 | 4% | 0 | 0% |
Undertaken in limited/exceptional circumstances only, e.g. feasibility studies | 1 | 4% | 0 | 0% |