So long as these differences are randomly distributed, they will wash out as ‘noise’ across large numbers of people
I think this is a crucial assumption that may not hold when comparing groups, i.e. there could be group differences (which could involve the same people, but before and after some event) in interpretations of the scales, due to differences in experiences between the two groups, e.g. a disability.
That immigrants to Canada seem to use the scales similarly to Canadians doesn’t mean they weren’t using the scales differently before they came to Canada. I think we actually discussed scale issues with life satisfaction on Facebook before (prompted by you?), and differences after adjusting for item responses seem to suggest different interpretations of the scale (or the items), or different relationships between the items.
Two examples (cited in one of the papers in your reading list, https://www.researchgate.net/publication/230603396_Theory_and_Validity_of_Life_Satisfaction_Scales):
Regarding interpersonal comparisons,
I think this is a crucial assumption that may not hold when comparing groups, i.e. there could be group differences (which could involve the same people, but before and after some event) in interpretations of the scales, due to differences in experiences between the two groups, e.g. a disability.
That immigrants to Canada seem to use the scales similarly to Canadians doesn’t mean they weren’t using the scales differently before they came to Canada. I think we actually discussed scale issues with life satisfaction on Facebook before (prompted by you?), and differences after adjusting for item responses seem to suggest different interpretations of the scale (or the items), or different relationships between the items. Two examples (cited in one of the papers in your reading list, https://www.researchgate.net/publication/230603396_Theory_and_Validity_of_Life_Satisfaction_Scales):
https://www.researchgate.net/profile/Robert_Biswas-Diener/publication/225502271_The_Divergent_Meanings_of_Life_Satisfaction_Item_Response_Modeling_of_the_Satisfaction_with_Life_Scale_in_Greenland_and_Norway/links/570184a808aea6b7746a7df9.pdf?origin=publication_list
http://www.people.virginia.edu/~so5x/IRT%20JRP%20revision%202.pdf
But there’s an obvious response here: we should use IRT to adjust for different scale interpretations.