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Table 1 DELTA2 recommendations undertaking a sample size calculation and choosing the target difference for a RCT

From: DELTA2 guidance on choosing the target difference and undertaking and reporting the sample size calculation for a randomised controlled trial

Begin by searching for relevant literature to inform the specification of the target difference. Relevant literature can:
 a. relate to a candidate primary outcome and/or the comparison of interest, and;
 b. inform what is an important and/or realistic difference for that outcome, comparison and population.
 2. Candidate primary outcomes should be considered in turn, and the corresponding sample size explored. Where multiple candidate outcomes are considered, the choice of the primary outcome and target difference should be based upon consideration of the views of relevant stakeholders groups (for example, patients), as well as the practicality of undertaking such a study with the required sample size. The choice should not be based solely on which yields the minimum sample size. Ideally, the final sample size will be sufficient for all key outcomes though this is not always practical.
 3. The importance of observing a particular magnitude of a difference in an outcome, with the exception of mortality and other serious adverse events, cannot be presumed to be self-evident. Therefore, the target difference for all other outcomes requires additional justification to infer importance to a stakeholder group.
 4. The target difference for a definitive (e.g. Phase III) trial should be one considered to be important to at least one key stakeholder group.
 5. The target difference does not necessarily have to be the minimum value that would be considered important if a larger difference is considered a realistic possibility or would be necessary to alter practice.
 6. Where additional research is needed to inform what would be an important difference, the anchor and opinion seeking methods are to be favoured. The distribution method should not be used. Specifying the target difference based solely upon a Standardised Effect Size approach should be considered a last resort though it may be helpful as a secondary approach.
 7. Where additional research is needed to inform what would be a realistic difference, the Opinion Seeking and the Review of the Evidence Base methods are recommended. Pilot trials are typically too small to inform what would be a realistic difference and primarily address other aspects of trial design and conduct.
 8. Use existing studies to inform the value of key “nuisance” parameters which are part of the sample size calculation. For example, a pilot trial can be used to inform the choice of the standard deviation value for a continuous outcome and the control group proportion for a binary outcome, along with other relevant inputs such as the amount of missing outcome data.
 9. Sensitivity analyses, which consider the impact of uncertainty around key inputs (e.g. the target difference and the control group proportion for a binary outcome) used in the sample size calculation, should be carried out.
 10. Specification of the sample size calculation, including the target difference, should be reported according to the guidance for reporting items (see below) when preparing key trial documents (grant applications, protocols and result manuscripts).