I have a question about how to interpret (differences in) the proportional similarity index Psi proposed by Bolnick et al. (2003) https://search.r-project.org/CRAN/refmans/RInSp/html/PSicalc.html

The question centers on the sensitivity of the index to the number of ‘diet items” that are used by individuals.

I am testing for differences in diet for individuals in a single population, either using ‘ingredients’ as input, or using derived ‘nutrients’ as input. For ‘ingredients’, there are up to 20 different items an individual can consume [e.g. ants, different berries and grains, egg, meat, etc] while ‘nutrients’ are limited to about 5 categories [protein, fat, carbohydrates, fiber, ash].

Are the resulting PSi values comparable? Say that an individual scores Psi = 0.1 based on ingredients but the corresponding nutrient-based Psi = 0.8, can I conclude that an individual acts as a specialist when it comes to ingredients but as a generalist for nutrients?

Or are these scores completely incomparable because of the different number of ‘items’ they are calculated on?

Thanks for any advice,

Diederik