arl_stat ★ India, 20201014 04:39 (14 d 03:08 ago) Posting: # 21996 Views: 248 

Greetings to all !! Hope all of you are safe in this pandemic situation. Mostly in public domain, the inter subject CV of drug is not mentioned. Either intrasubject cv or confidence interval ranges are mentioned. So can we use this intrasubject CV to estimate sample size for parallel study design? Is there any formula to calculate intersubject CV using 90% confidence interval ranges and number of subjects? First can we calculate the sample size of 2 way crossover study design and then double the subjects and finalize the sample size for parallel study design? Please suggest. Stay Safe. Thank You Edit: Category changed; see also this post #1. Subject line changed; see also this post #2. [Helmut] 
Helmut ★★★ Vienna, Austria, 20201014 09:00 (13 d 22:47 ago) @ arl_stat Posting: # 21998 Views: 221 

Hi arl_stat, » Mostly in public domain, the inter subject CV of drug is not mentioned. You fell into trap of wrong terminology. Even if a study was performed in a parallel design, one gets the total (a.k.a. pooled) CV – not the intersubject CV.$$CV_{\textrm{total}}=\sqrt{\exp\left(\log_{e}(CV_{\textrm{inter}}^{2}+1)+\log_{e}(CV_{\textrm{intra}}^{2}+1)\right)1}\tag{1}$$ Simply: The total variance (which you observe in any design) is the sum of inter and intrasubject variances.$$s_{\textrm{total}}^{2}=s_{\textrm{inter}}^{2}+s_{\textrm{intra}}^{2}\tag{2}$$ The intersubject CV is only accessible in a crossover study (see this post). » […] can we use this intrasubject CV to estimate sample size for parallel study design? No. » Is there any formula to calculate intersubject CV using 90% confidence interval ranges and number of subjects? No, since you need the CV_{total} calculated acc. to \((1)\) from a crossover – or directly observed in a parallel design – and there is no relationship between CV_{inter} and CV_{intra}. Hence, CV_{intra} from a crossover alone does not help. See also this post and followings. » First can we calculate the sample size of 2 way crossover study design and then double the subjects and finalize the sample size for parallel study design? No (see above). For ages rumors are swirling around a factor of two but that’s plain nonsense. Think about a drug with low CV_{intra} and polymorphic metabolism. Then the factor can easily be >5 and by doubling the sample size of a crossover the parallel study will be grossly underpowered. BTW, in rare cases CV_{intra} > CV_{inter}. Run a – not too small! – pilot study or a TSD.*
— Diftor heh smusma 🖖 Helmut Schütz The quality of responses received is directly proportional to the quality of the question asked. 🚮 Science Quotes 
arl_stat ★ India, 20201014 13:02 (13 d 18:45 ago) @ Helmut Posting: # 22001 Views: 156 

Thank you so much sir for your quick response and guidance. 