2 Methods

We drafted this manuscript using the PRISMA extension for reporting literature searches in systematic reviews (Rethlefsen et al., 2021), the PRISMA Extension for Scoping Reviews (Tricco et al., 2018), and JBI guidance on scoping reviews (Peters et al., 2020). We prospectively registered the review protocol on the Open Science Framework (https://osf.io/xcthk).

2.1 Eligibility Criteria

We included empirical studies examining the 4DSW at primary and secondary schools in the United States (see Table 1). We excluded studies that focused on variations in traditional school calendars in the United States other than reducing the traditional five-day school week (5DSW) to a 4DSW (e.g., extending the length of the school year). We also excluded studies in pre-school or postsecondary school, as well as studies taking place outside of the United States. The population of interest is any stakeholder relevant to primary and secondary schools in the United States (including students, parents, teachers, and other school staff). We included all research designs that involved the collection and analysis of empirical data; we also included studies that used computational modeling to simulate empirical data. We excluded conceptual papers, opinion articles, narrative literature reviews, and non-research sources (e.g., news articles). We included grey literature unless we could not obtain the full-text (e.g., conference abstracts, unavailable dissertations). Due to limitations in resources and language proficiency, we excluded studies published in a language other than English.

2.2 Information Sources

We searched for eligible studies on 23 May 2023 using the following electronic databases (with search platforms in parentheses): PubMed (National Library of Medicine), ProQuest Dissertations & Theses A&I (ProQuest), and Social Science Premium Collection (ProQuest). The Social Science Premium Collection includes the Criminology Collection, Education Collection (which includes Education Resources Information Center, or “ERIC”), International Bibliography of the Social Sciences, Library & Information Science Collection, Linguistics Collection, Politics Collection, Social Science Database, and Sociology Collection. We did not contact authors nor search study registries, web search engines, specific web sites, or journal tables of contents.

2.3 Search Strategy

A copy of the full search strategy is available on the Open Science Framework (https://osf.io/ryva8/). We developed the search strategy using the Peer Review of Electronic

Search Strategies (PRESS) Guideline (McGowan et al., 2016) and terms common in studies identified in a preliminary scan of the literature on Google Scholar. The search strategy did not involve the use of any published search filters. We uploaded the citations into a web-based systematic review data management software (DistillerSR). We used DistillerSR to detect and remove duplicate citations prior to the process of screening citations for eligibility.

2.4 Selection Process of Sources of Evidence

A copy of the selection process codebook is available on the Open Science Framework (https://osf.io/ryva8/). We used standardized forms in DistillerSR and did not blind selectors to journal titles or study authors and their institutions. First, one PhD-level reviewer (SG or ED) and at least one undergraduate reviewer (BC, SH, or SM) screened the title and abstract of each citation for potentially eligible studies. Then, we retrieved full texts for each citation deemed potentially eligible. One PhD-level reviewer (SG or ED) and at least one undergraduate reviewer (BC, SH, or SM) assessed each full-text for eligibility, recording reasons for excluding citations. We used the PRISMA2020 R package (Haddaway et al., 2022) to visually summarize the search and selection process via a flow diagram with the number of records screened, assessed for eligibility, and included in the scoping review (Page et al., 2021; Tricco et al., 2018).

2.5 Data Charting Process and Items

Charting the results involved an iterative process of continually updating the charting items to ensure that we extracted all relevant information (Peters et al., 2020; Tricco et al., 2018). A copy of the final data charting codebook is available on the Open Science Framework (https://osf.io/yrk2w/). One PhD-level reviewer (SG or ED) and at least one undergraduate

reviewer (BC, SH, or SM) independently collected data from each eligible article using DistillerSR. We resolved any disagreements through discussion. We collected bibliographic information (e.g., authors, year published), information related to the review question (i.e., concept, context, population), and methodological characteristics (e.g., research design). As the purposes of this scoping review is to provide an overview of the existing evidence regardless of methodological quality or risk of bias, we did not critically appraise the included sources of evidence (Tricco et al., 2018).

2.6 Synthesis of Results

We used the GRADE Evidence to Decision (EtD) framework to group and summarize the types of empirical research evidence available on the 4DSW (Alonso-Coello et al., 2016). We described key parameters (characteristics of the sources of evidence and the types of empirical research evidence) through simple frequency counts.