Why are kit codes not given out in list order?

Kit codes are used in blinded trials to keep the true treatment group secret. The code list links kit codes to treatments, for example:

Sequence Kit code Treatment Kit block
1 AF5 Active 1
2 GY6 Placebo 1
3 YU1 Placebo 1
4 FT2 Active 1
5 LP9 Placebo 2
6 HX3 Active 2
7 ...

However, this list does not determine the order that treatments are assigned. Instead, treatments are chosen by the randomisation method, such as minimisation or a randomisation list. Once the treatment has been chosen a kit code that matches the treatment is found in the code list – see this page for an example.

Although it is possible to use the code list to determine the next treatment (i.e. take the place of a randomisation list), at Sealed Envelope we don't do this. This is because having a separate code list is more flexible and allows:

  • A different randomisation method to be used (such as minimisation) in blinded trials
  • Additional kit codes to be allocated after randomisation, such as to replace a lost kit or for re-supply at follow-up visits
  • Different kit types to be chosen depending on other factors at randomisation (such as a higher dose for heavier participants)
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