Step three - continued

Selection bias

To avoid selection bias the idea of randomisation to generate groups that are comparable in terms of known or unknown potential confounding factors is used. Randomisation consist of two parts:

1. generation of allocation sequences
2. concealment of allocation sequences

Examples of adequate generation of allocation sequences, in other words, where the resulting sequences are unpredictable are computer generated random numbers, drawing lots, tossing a coin or throwing a die.

Inadequate generation of allocation sequences would include: organisation according to case record number, date of birth, date of admission or alternation.

Examples of adequate concealment of allocation sequences, this is where the enrolling investigators cannot foresee assignment are: a priory of numbered or coded drug containers prepared by an independent pharmacy, central randomisation performed at a site away from the trial location, sequentially numbered, sealed, opaque envelopes.

Examples of inadequate concealment of allocation sequence where the enrolling investigators can foresee upcoming assignment are: all procedures based on inadequate generation of allocation assignment, open allocation schedule, unsealed or non-opaque envelopes.

Concealment has been shown to be more important in preventing bias than other components of allocation, such as generation of the allocation sequence.