Sex-by-treatment interaction [Design Issues]

posted by Helmut Homepage – Vienna, Austria, 2023-05-05 14:44 (296 d 02:41 ago) – Posting: # 23544
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Hi BEQool,

❝ […] is Gender*Treatment also a between-subject factor (like Sequence and Gender here)?

If you replace gender by sex, yes. Muse about it. ;-) BTW, you could even drop sequence from the model

❝ In the article Evaluation of sex-by-formulation interaction in bioequivalence studies of efavirenz tablets by González-Rojano et al. it doesnt say anything about which factor they used (within- or between-subject) to test Gender*Treatment interaction.

See p. 1731, right column, top. That’s the model proposed by the FDA earlier:*$$\eqalign{Y&|\;\text{sequence},\,\text{sex},\,\text{sequence}\,\times\,\text{sex},\\
&\phantom{|}\;\text{treatment},\,\text{sex}\,\times\,\text{treatment}}$$ I employed it also in my meta-analysis. The ‘purpose’ of this model is only to assess the interaction. Since treatment appears twice in the model, it is not possible to get an unbiased estimate of it (for a similar story see this article).

❝ Additionally, how do you know which factor is a within-subject and which is a between-subject? If anyone has an explanation in simple words :-)

In simple words? I’ll try. For a given subject anything which remains constant throughout a study leads to a between-subject factor (e.g., sex, sequence, stage, group, site). Everything else leads to a within-subject factor (e.g., treatment, period).

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