STAR★Methods

Key resources table

REAGENT or RESOURCE | SOURCE | IDENTIFIER
Software and algorithms

Microsoft Word | Microsoft | https://www.microsoft.com/da-dk/microsoft-365/word?market=dk
EndNote | Clarivate | https://endnote.com/

Method details

This study employed a three-phase mixed-methods approach combining a structured expert consultation conducted through an in-person workshop, a subsequent literature analysis, and collaborative synthesis to identify and characterise knowledge gaps for plastics governance across the life cycles and value chains of plastics. While expert elicitation is often used to estimate uncertainties for formal modeling130 related approaches can also be used to explore qualitative issues for which current evidence is limited.131 The paper did not involve empirical research on human subjects or the collection of personal data, rather the workshop functioned as a scholarly expert consultation informing a subsequent author-led literature-based analysis.

In the first phase, we convened a transdisciplinary expert workshop during a full day in January 2024. The workshop brought together 24 experts working in Europe from diverse fields including environmental science and policy, economics, and social sciences, representing academic institutions, public policy, and civil society organisations. Experts were invited based on their published research contributions and practical experience in plastics-related policy and governance aiming to capture subject-matter expertise in key topics connected to plastics (ecotoxicology, environmental health, climate change mitigation, industrial change) as well as experts on environmental governance (public and private governance initiatives, policy implementation and evaluation). All workshop contributors were invited to be part of the follow-up process and contribute to this paper. Fifteen contributors accepted and have co-authored the paper and nine contributors declined the invitation. Their contributions are acknowledged in the acknowledgments section where individuals who consented to being named are listed, while others are referred to in aggregate. To provide additional transparency regarding the composition of the workshop contributors, an overview of institutional affiliations and the geographic scope of represented institutions is presented in aggregated form in the table below.

Dimension | Category | Number of experts
Institutional affiliation | academic institution | 11
applied research institution | 2
EU institutions and agencies | 8
national public administration | 1
civil society organisation | 2
Geographic scope of institutions | EU-based | 21
non-EU Europe | 3

Prior to the workshop, the invited experts submitted key publications and documents deemed relevant for understanding the current state of plastic governance and key knowledge gaps for effective policymaking on plastics. Based on these submissions, the workshop utilised a structured format to elicit expert knowledge in further identifying knowledge gaps. The workshop was divided into three sessions focusing on key themes emerging from the pre-workshop process: (i) existing initiatives and instruments across issue domains, (ii) data, monitoring, and reporting systems, and (iii) emerging multi-scalar governance approaches. The experts were divided into thematic groups in three sessions, each group aiming to identify knowledge gaps relating to their core expertise. The division into smaller groups was part of aiming to capture a wide variety of perspectives on the issue as well as to reduce the risk for individual dominance during the workshop. Workshop discussions were documented to capture collectively identified themes and knowledge gaps, which served as inputs to the subsequent literature-based analysis conducted by the author team. At the end of the workshop 21 potential knowledge gaps had been identified, which served as a starting point for the subsequent author-led analysis. The identification and framing of knowledge gaps are inherently shaped by the expertise and perspectives represented in the workshop and subsequent analysis, reflecting the expert-informed nature of the approach.

Following the workshop, an expert-informed literature review was conducted to validate and expand upon the identified knowledge gaps with the best available knowledge. The author group was divided into four working groups, each covering knowledge gaps relating to one part of the life cycle of plastics (production, use, end-of-life) as well as one group focused on cross-cutting issues. Three authors focused on production, four on use (five including one author contributing to two groups), four on end-of-life, and four on cross-cutting issues.

The authors reviewed recent peer-reviewed articles within their domains of expertise as well as relevant gray literature from international, policy-relevant organisations to capture practice-oriented evidence and insights from ongoing policy processes. Papers and reports included in the review were identified using a combination of iterative, purposive searches in scholarly databases and institutional repositories (e.g., UN iLibrary and OECD iLibrary) as well as citation tracking. Search strategies were developed iteratively within each working group and guided by the thematic focus of the identified knowledge gaps. As a result, the search process was not exhaustive or conducted using a formal systematic review protocol with predefined search strings and inclusion criteria that enable full reproducibility in the manner of systematic reviews. Instead, it was guided by thematic relevance and expert judgment, reflecting a deliberate trade-off to capture a broad and evolving evidence base across disciplines and source types, including materials not systematically indexed in scholarly databases. The working groups conducted detailed analysis of the literature within their respective themes to characterise knowledge gaps according to their nature, implications for governance, and possible policy responses. This framework enabled us to identify cross-cutting issues and interconnections between different life cycle stages.

The third phase consisted of collaborative synthesis and validation. Working group leaders jointly reviewed the contributions of the working groups to ensure consistency and coherence, leading to the merging of similar knowledge gaps across some life cycle stages and resulting in the list of 13 knowledge gaps presented in the paper. The set of identified knowledge gaps was clustered into four strategic action areas to support synthesis and policy relevance. This clustering was first done by the three working group leaders. The clustering followed an iterative and interpretive approach, in which knowledge gaps were grouped based on their shared implications for governance and the types of interventions required to address them, rather than strictly by life cycle stage. The draft manuscript, including the final list of knowledge gaps and the identified action areas, was circulated twice among all authors for review and refinement. This approach allowed us to comprehensively map the landscape of knowledge gaps and identify strategic action areas while maintaining scientific rigor through multiple validation steps.

References were managed using EndNote (Clarivate), and the manuscript was prepared using Microsoft Word (Microsoft).

Quantification and statistical analysis

This study did not involve statistical hypothesis testing or quantitative data analysis. The study employed a qualitative, expert-informed research design combining a structured expert workshop, literature analysis, and collaborative synthesis to identify and characterize knowledge gaps for plastics governance. No statistical software was used.