· Standard deviation of the outcome measure, which is needed in some cases to estimate sample size

· Willingness of participants to be randomised

· Willingness of clinicians to recruit participants

· Number of eligible patients, carers or other appropriate participants

· Characteristics of the proposed outcome measure and in some cases feasibility studies might involve designing a suitable outcome measure

· Follow-up rates, response rates to questionnaires, adherence/compliance rates, ICCs in cluster trials, etc.

· Availability of data needed or the usefulness and limitations of a particular database

· Time needed to collect and analyse data