All relevant articles were screened according to specific inclusion criteria.
Even though our search was extensive, we cannot be sure that all relevant articles were located.
First, it is impossible to be certain that we obtained all relevant articles.
All relevant articles indexed in PubMed, Springer, Science Direct, and Scopus with no limitations from 1961 to 2016 were searched.
The "All Fields" search query was utilized to provide the best opportunity to capture all relevant articles for our review.
A comprehensive search of major databases through PubMed, EMBASE, and Cochrane CENTRAL was conducted to locate all relevant articles published from inception to October 2014.
In total, 231 papers were retained following second phase quality evaluation, and only 152 (20% of all relevant articles) met all ranking criteria.
We extended the search to articles containing text words "headache care*.tw" or "headache service*.tw" in order to ensure identification of all relevant articles.
A search of articles using the MEDLINE database and PubMed (National Library of Medicine) was performed to identify all relevant articles published in the area.
Lastly, inherent to any systematic review, there is the possibility that not all relevant articles were identified through the used search terms and literature review.
Three authors (ELA, SAR, and RHM) reviewed the full text of all relevant articles using a standardized form to extract individual quality metrics and study details.
