Bioequivalence and Bioavailability Forum

Dear youri!

❝ ❝ Remark: even if the CV is not given, you can calculate it from the confidence limits and the sample size of a published study by means of a little bit of algebra

❝

❝ Can you please give some more details on the "little bit of algebra"?

No sorry, such calculations are part of my professional services.
Although I really love public discussions, such an information would go beyond one point – the one I’m making part of my living from.

❝ How exactly can I calculate the CV on the basis of a published CI and sample size?

If you solve the little ‘algebra’, and the study was balanced (equal number of subjects in both sequences), results are exact. The only limitation is the number of significant digits the confidence interval was reported, e.g., if the CI is given as 89.0–115% (n=24), you will get the CV also to three significant digits as 26.3%
If the CI was reported as 89–115%, I apply a conservative approach of using 88.5–115.4% instead (thus avoiding any potential rounding errors from the report), getting 27.3%.

If the study was imbalanced, calculations yield only an approximate result, but the deviation from exact values generally is not very large (however, conservative).
You can obtain an exact result if the number of subjects in each sequence is known – which is rarely stated in publications.
Example: (CI 89.0%-115%; going from balanced 12/12 to increased imbalance, keeping the total sample size at 24):

n1/n2 | CVintra
------+-------
12/12 | 26.29%
13/11 | 26.20%
14/10 | 25.91%
15/ 9 | 25.43%
16/ 8 | 24.74%

In other words if the study was extremely imbalanced (16/8 = CV 24.7%) and you assume a balanced design (12/12 = CV 26.3%), you are on the safe side.