## Welch’s t-test [Regulatives / Guidelines]

Hi Imph,

Correct. Specifically stated by the FDA

Perform Welch’s

Calculate the confidence interval as usual with the

It’s also easy in Phoenix WinNonlin and PKanalix. Consult the respective manual. You should check whether your setup is correct.*

❝ I would like to know if in the parallel design, we should not assume the equality of the variances.

Correct. Specifically stated by the FDA

*For parallel designs* […] *equal variances should not be assumed.*

*The precise model to be used for the analysis should be pre-specified in the protocol. The statistical analysis should take into account sources of variation that can be reasonably assumed to have an effect on the response variable.*

❝ […] how do the statistical model, the statistical analysis and the formula for the confidence interval become?

Perform Welch’s

*t*-test,*i.e.*, with Satterthwaite’s approximation of the degrees of freedom. $$\nu=\frac{\big{(}\frac{s_1^2}{n_1}+\frac{s_2^2}{n_2}\big{)}^2}{\frac{s_1^4}{n_1^2(n_1-1)}+\frac{s_2^4}{n_2^2(n_2-1)}}$$where \(\small{n_1,n_2}\) are the number of subjects under treatments T and R, respectively. \(\small{s_1,s_2}\) are the standard deviations of treatment arms. Not by any chance it is the default in and**SAS**

, whereas **SPSS**

calculates always the conventional *t*-test (assuming equal variances)*and*the Welch-test. In the output table, use the second row.Calculate the confidence interval as usual with the

*t*-value for given \(\small{\alpha,\,\nu}\).It’s also easy in Phoenix WinNonlin and PKanalix. Consult the respective manual. You should check whether your setup is correct.*

- Fuglsang A, Schütz H, Labes D.
*Reference Datasets for Bioequivalence Trials in a Two-Group Parallel Design.*AAPS J. 2015; 17(2): 400–4. doi:10.1208/s12248-014-9704-6. PMID 25488055. Free Full text.

—

Helmut Schütz

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*Dif-tor heh smusma*🖖🏼 Довге життя Україна!_{}Helmut Schütz

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### Complete thread:

- Unequal variances in parallel design Imph 2022-06-20 16:16 [Regulatives / Guidelines]
- Welch’s t-testHelmut 2022-06-20 16:47
- Welch’s t-test Imph 2022-06-23 13:02
- Welch’s t-test Helmut 2022-06-23 14:19
- Welch’s t-test Imph 2022-06-26 10:06
- Read the paper, use the code(s)! Helmut 2022-06-26 12:12

- Welch’s t-test Imph 2022-06-26 10:06

- Welch’s t-test Helmut 2022-06-23 14:19

- Welch’s t-test Imph 2022-06-23 13:02

- Welch’s t-testHelmut 2022-06-20 16:47