This systematic review will be conducted according to the recommendations of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) checklist. The protocol of this systematic review has been registered in PROSPERO (No. CRD42020180887)
Eligibility Criteria
Study types
All primary studies that have reported the prevalence of NSSI will be eligible to be included. This includes observational, cohort, cross-sectional, correlational studies that have investigated NSSI prevalence in the general population. The studies with any definition of self-harm will be eligible. There will be no language restriction. Those studies that determine NSSI prevalence in specific population groups such as natives, minorities, or among patients with particular psychiatric disorders will not be eligible.
Population
Studies with the subjects of any age group (children, adolescents, youth, and adults) and of any gender (male or female) and any nationality will be eligible to be included.
Definitions
This systematic review will determine point prevalence, annual prevalence, and lifetime prevalence of NSSI. The therapeutic interventions to manage NSSI will not be considered. Studies with any definition used to describe self-harm behavior (mentioned earlier in the introduction part) will be eligible. Any study that has used the diagnostic criteria provided by the DSM-5 to describe self-harm behavior will be eligible.
Sampling methods
Studies with a minimum of 25 subjects with any type of sampling (random or non-random sampling) will be eligible.
Search strategy
Electronic databases
The electronic databases that will be used to conduct the search include PubMed, PsycINFO, MEDLINE, Embase, Scopus, ProQuest, Web of Science and Google scholar. The search will be conducted in the time period from 1990 to 31/12/2019.
The appropriate keywords were determined using controlled vocabularies of MeSH (Medical Subject Headings) and Emtree as well as the three-step search method. The keywords include the following:
(Self-Injurious Behavior) OR (self-injury) OR (self-destructive Behavior) OR (self-harm) OR (self-mutilation) OR (NSSI) OR (self-inflicted wound) OR (self-inflicted injury) OR (self-poisoning) OR (skin-cutting) OR (parasuicide) OR (self-cutting)
The search syntax developed to be used for PsycINFO database is presented in additional file 2. This search syntax will be modified and will be used for other electronic databases.
Other sources
In addition to electronic databases, other sources such as articles, conferences, dissertations, previous reviews studies, key journals, reference list of previous reviews, and reference list of the included studies will be considered.
Screening and selection
After completing the systematic search of the mentioned bibliographic databases, the search results will be entered into the Mendeley reference manager program. Then, two reviewers will screen the search results separately and independently. In the screening process, using titles and abstracts of the citations, the search results will be categorized as relevant, irrelevant, or indeterminate. Any disagreement will be resolved through discussion and consensus, and finally if disagreement remains, a third reviewer will be consulted.
At the next step, the full-texts of the relevant as well as indeterminate studies will be reviewed by two reviewers and considering the inclusion and exclusion criteria, they will be selected. In case of any disagreement regarding meeting inclusion and exclusion criteria, a third reviewer will be consulted. There will be no language limitation and in case of languages other than English, the full text of the citation will be translated into English.
Methodological Quality Assessment
To assess the methodological quality of the primary articles, a form will be used. This process will be done by two reviewers independently. The reviewers will be asked to fill out the required form for each article separately. Any disagreement will be resolved through discussion and consensus and finally if disagreement remains, a third reviewer will be consulted.
Data extraction and Data synthesis
Data extraction process will be accomplished independently by two reviewers using a designed data collection form. The required data that will be collected include the following:
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Study identification data (first author, publication date, journal name, type of the study, location)
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Background data (age, gender, number of participants studied, specific population groups, inclusion and exclusion criteria of the studies, education level of the participants)
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Data related to the primary objective of the systematic review (NSSI prevalence rate), secondary objectives (prevalence of NSSI based on different definitions, types, and motives of self-harm, gender, and age groups)
The main data that will be used for meta-analysis includes denominator of a proportion (the number of subjects diagnosed with NSSI to the total number of subjects). If the required data are not presented appropriately in the article, the figures will be reviewed. If the data are presented in a figure, we will use online WebPlotDigitizer tool to extract the required data.
In case that the required data are not presented in the text or in the figures, we will contact the corresponding author of the article via email (maximum of three times every 7-10 days). If the authors do not reply and provide the required data, that particular article will be excluded.
The main size in this study is the prevalence, and meta-analysis will be conducted only if there are at least 4 studies.
The main data of meta-analysis will be consisted of the denominator of the prevalence measure (The numerators of affected people to the total number). If the studies presented details on the prevalence with respect to gender, age, kind of primary relationship and kind of extra-dyadic behavior, the information will be investigated in the same basic form.
The meta-analysis of prevalence estimates will be accomplished using “metaprop” command by Stata (Ver. 13.0, StataCorp, College Station, TX, US) statistical package. In order to determine the combination model, considering similarities and differences between the primary studies, fixed-effects or random-effects model will be applied to summarize the prevalence rates. In addition, forest plots will be used to depict the prevalence estimates.
In order to determine heterogeneity between the results of the primary studies, the Cochran’s Q test (P values less than 0.05 indicate notable variability) and inconsistency index (I2) will be used. Based on categories proposed by Higgins et al. to define heterogeneity, I2 values of 0 to 24.5% indicate low heterogeneity, values of 25% to 49.5% indicate moderate heterogeneity, and values between 50 and 74.5% indicate severe heterogeneity, and values higher than 75% will indicate very severe heterogeneity.
In order to assess the publication bias, the following methods will be used sequentially:
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Inspection of funnel diagram: if the total number of the included studies is less than 10, this method will not be used. In case of asymmetry in the funnel plot, the next two methods will be used.
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In the second step, the nonparametric Begg's test and parametric Egger's test will be used. If P value of any of these tests is less than 0.1, then the next method will be used.
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The third method is “fill and trim” method which identifies and corrects asymmetry of the funnel plot.
In order to measure quality of evidence, the GRADE (Grading of Recommendations Assessment, Development and Evaluation) tool will be used. In order to perform sensitivity analysis, “one-out remove” method will be applied using the “metaninf” command by Stata software. In addition, in order to determine the effect of methodological quality on the combined results, “subgroup analysis” or “meta-regression” will be used. In case of significant differences are observed regarding the prevalence estimates of NSSI between high-quality and low-quality studies, the final results will be presented using the results of high-quality studies.