A Cohen’s D is a standardized effect size which is defined as the difference between your two groups measured in standard deviations. Because the Cohen’s D unit is standard deviations, it can be used when you have no pilot data. As a general guide a Cohen’s D of 0.3, 0.5 and 0.8 corresponds to mild, moderate and large effect sizes, respectively. However, this depends on the context; for example biomarkers would want the effect size to be at least 2, as biomarkers aren’t useful when the groups overlap.

Example paragraph:

As no pilot data has been collected for these experiments, a standardized effect size was used for the power analysis. For this pilot study we will be aiming to detect a large clinically relevant effect size with a Cohen’s d of 0.8. A power analysis using the two-tailed student’s t-test, Sidak corrected for 3 comparisons, with an alpha of 0.05 and a power of 0.8 was performed. From this analysis it was found that 35 human samples in each group would be required.

Cohen J. 1988. Statistical Power Analysis for the Behavioral Sciences. 2nd ed.Hillsdale: Lawrence Erlbaum Associates.