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University of Colorado Denver


Sample Size Justification

When to contact us

Contact at the earliest possible time, usually more than 6 weeks prior to any deadline. This time frame is needed because it is often time consuming for you to find the needed information for the calculations.

What to expect

After a scope of work is developed and signed, be prepared to spend much of a first meeting with the collaborating biostatistician discussing your science and study design before moving onto a discussion on sample size. The end product is a table of sample size choices and a written paragraph justifying the sample size.

Key data needed from you

Sample size is a function of:

  1. variation in outcome measures (and sometimes primary predictors),
  2. the size of a clinically meaningful difference between groups,
  3. level of significance (are multiple tests being performed),
  4. and the desired type II error (the probability that you will not find a difference when a difference, in truth, exists).

You will need to provide from historical literature, pilot data, or other clinical information the following information.

  • Measures of variation (standard deviations) of the outcome measures.
  • An estimate of a clinically meaningful difference between groups or an estimate of the size of association that is clinically meaningful (e.g., correlation).
  • In studies with repeatedly collected measurements on an individual (or cluster) estimates of correlation within individuals (or clusters) is also needed.
  • For studies with a continuous variable of interest (e.g., age) the variation of the variable of interest is also needed.

The lead biostatistician cannot perform a valid literature search for you. Finding this information is something that only those with scientific expertise in the field can adequately do.

Colorado Biostatistics Consortium

13001 17th Place | Mail Stop B119 | Room W3129, Building 500 | Aurora, CO 80045
303-724-4370 |

Request Biostatistics | Department of Biostatistics and Informatics | BERD | BBSR | ColoradoSPH

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