Welch’s t-test [Regulatives / Guidelines]

posted by Helmut Homepage – Vienna, Austria, 2022-06-20 16:47 (11 d 09:05 ago) – Posting: # 23070
Views: 370

Hi Imph,

» 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.

and in between the lines by the EMA

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 [image] 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 [image] 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.*

Dif-tor heh smusma 🖖 [image]
Helmut Schütz

The quality of responses received is directly proportional to the quality of the question asked. 🚮
Science Quotes

Complete thread:

UA Flag
 Admin contact
22,181 posts in 4,647 threads, 1,572 registered users;
online 2 (0 registered, 2 guests [including 2 identified bots]).
Forum time: Saturday 01:53 CEST (Europe/Vienna)

Reach for the stars,
even if you have to stand on a cactus.    Susan Longacre

The Bioequivalence and Bioavailability Forum is hosted by
BEBAC Ing. Helmut Schütz