2.1 Search strategy
Two independent researchers searched online international databases, including Science direct, ISI, Pubmed, Scopus, Embase, and Google scholar, to determine relevant studies published between 2000 and Jan 2020. Appropriate keywords were used: “Human papilloma virus”, “Oral cancer”, “Iran”, and “Oral squamous cell carcinoma” which were combined with and/or/not. In addition, to improve the search sensitivity, we examined the references of these studies. Two other researchers assessed the search approach at random and confirmed that all appropriate studies had been detected. Moreover, additional efforts have been made to identify unpublished studies.
2.2 Study selection
During the advanced search, full texts or abstracts of all articles were reviewed. First, duplicate papers were excluded from the study. Then, irrelevant papers were deleted after analyzing the titles, abstracts, and full texts.
2.3 Quality assessment
In order to assess the quality of the studies the STROBE statement was used. The statement is a checklist of 22 items covering all components of the methodology, including sample size, type of study, methods and instruments for data collection, variables, aims, study objectives, study population, results presentation, and statistical analysis. In current study, the lowest and highest scores were 0 and 44, respectively. The studies were divided into three groups, based on the quality analysis: low quality (< 15.5), average quality (15.5- 29.5), and high quality (30-44).
2.4 Inclusion criteria
All papers that passed the above assessment phases for high quality scores were selected if meeting the following conditions: 1) Case- control studies published in English and 2) Case-control studies on the prevalence of HPV infection in patients with oral cancer.
2.5 Exclusion criteria
The following studies have been ruled out: 1) Case reports or case series. 2) Articles with no access to the full-text. 3) Duplicated studies. 4) Conference abstracts with no full-text publication available. 5) Studies published in languages other than English. 6) Studies with low and average quality scores.
2.6 Data extraction
After selection of suitable studies, the following data were extracted: name of authors, year of publication, place where the study was conducted, study size, number of cases and controls, source of samples, total infection prevalence among oral cancer patients, and type of HPV.
2.7 Statistical analysis
In this study, the degree of heterogeneity in studies results was measured using Cochran’s Q test and I2 index. The odds ratio of the oral cancer was estimated based on a random-effect model. Also, sensitivity analysis was used for detection of heterogeneity. We also designed forest plots for estimation of odds ratio of oral cancer according to the results of primary studies with 95% confidence intervals (crossed lines). Each box in a forest plot indicated the weight of the study. For measuring the publication bias, the Funnel plot was used by subjective judgment in each study. In this study Trim and Fill method was used to confirm the validity and reliability of the results. All statistical analyses were performed in Stata SE, V.11 software.