In this study, we concentrate our effort on re-assessing keyphrase extraction performance on three increasingly sophisticated levels of document preprocessing described below. 
 Question: what keyphrase extraction models were reassessed?
Answer with content missing: (LVL1, LVL2, LVL3) 
- Stanford CoreNLP
- Optical Character Recognition (OCR) system, ParsCIT 
- further abridge the input text from level 2 preprocessed documents to the following: title, headers, abstract, introduction, related work, background and conclusion.