Two main research studies have been carried out on the use of the PLUS model in schools and there are some ongoing studies. The following links will provide you with an abstract of the studies and links to publications where appropriate.
Herring, J (2000) Theory into practice - using the PLUS model to teach information skills and support the curriculum in a secondary school in Howe, E (ed) Do you read me? Fourth International Forum on Research in School Librarianship. Malmo, Sweden
Herring, J E, Tarter, A-M and Naylor, S (2002) An evaluation of the PLUS model to develop pupils' information skills in a secondary school School Libraries Worldwide 8(1) January 2002. 1-24