  Conducting a systematic review (SR) is comprised of multiple tasks: (i)
collect documents (studies) that are likely to be relevant from digital
libraries (eg., PubMed), (ii) manually read and label the documents as relevant
or irrelevant, (iii) extract information from the relevant studies, and (iv)
analyze and synthesize the information and derive a conclusion of SR. When
researchers label studies, they can screen ranked documents where relevant
documents are higher than irrelevant ones. This practice, known as screening
prioritization (ie., document ranking approach), speeds up the process of
conducting a SR as the documents labelled as relevant can move to the next
tasks earlier. However, the approach is limited in reducing the manual workload
because the total number of documents to screen remains the same. Towards
reducing the manual workload in the screening process, we investigate the
quality of document ranking of SR. This can signal researchers whereabouts in
the ranking relevant studies are located and let them decide where to stop the
screening. After extensive analysis on SR document rankings from different
ranking models, we hypothesize 'topic broadness' as a factor that affects the
ranking quality of SR. Finally, we propose a measure that estimates the topic
broadness and demonstrate that the proposed measure is a simple yet effective
method to predict the qualities of document rankings for SRs.
