The results of this study are based on both interviews and observations of the participants.
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AlpheccaStarsThe results of this study are based on both interviews and observations of the participants.To my ear, it's better to keep "observation" in the singular (uncountable).
AvangiTo my ear, it's better to keep "observation" in the singular (uncountable).When I read it in the plural it seems to suggest that the results are based in part on "observations" which the participants have made.That's right.