Table 1

Study selection criteria

Inclusion criterion	Rationale
Primary, published, peer-reviewed studies	Restricting the synthesis to primary, published, peer-reviewed studies matches the aims of examining primary evidence.
Studies examining understanding	Understanding is defined as objectual understanding: understanding of something, such as collection of ideas or a subject matter71. Studies examining participants’ knowledge, perceptions, sentiments, values or experiential understanding will be included. This reflects that understanding can be developed through experiential learning72, emotional learning73 as well as abstract learning.
Among patients and/or the public	The synthesis will examine understanding among both patients and the public. People regularly transition between being one or the other17, and make diagnostic and screening decisions drawing on understanding they developed overtime and in either role. So, it is appropriate to examine understanding of OverTD among both groups. It will be distinguished whether studies are about patients, the public or both. People have a differing engagement with health decision making when they are patients or the public74. To account for this, synthesis results for each group will be compared, and important intergroup differences will be considered in study outcomes.
Of overtesting and/or overdiagnosis (OverTD)	Studies about both overtesting (OT) and overdiagnosis (OD) will be included, as both are deeply interlinked and underpinned by common broader patient ideas about healthcare. However, understandings of OT and OD may differ. To account for this, studies will be classified based on whether they examine OT, OD or both. The synthesis results will be compared by these classifications, and important differences will be accounted for. Synonymous concepts to overtesting and overdiagnosis will be included, such as ‘over-detection’ and ‘overuse of diagnostic testing’. The term ‘overdiagnosis’ was popularised relatively recently75, but it was predated by earlier terms76, and it is important to capture these earlier studies. Studies which did not explicitly aim to understand how to inform patients or the public about OverTD are outside the scope of this synthesis and will be excluded. Studies that only address overtreatment and not overtesting or overdiagnosis will also be excluded.
Either qualitative or mixed-methods study design	Mixed-methods studies will be included where their qualitative components can still be examined in the thematic meta-synthesis. Quantitative components of mixed-methods studies will be excluded, as will studies where it is not possible to differentiate between quantitative and qualitative components of analysis.
Published in the English language	Only English language studies will be included, as the authors are English speakers, and relying on translations of non-English studies could introduce inaccuracies into the analysis.
Published in any year	There will be no date restrictions: older insights may still be relevant.
Conducted in any setting	There will be no setting restrictions: studies from all settings may potentially contain transferable insights about patient and public understandings of OverTD.
Focusing on the general concepts of OverTD and/or in relation to any condition/s or interventions	While patient and public understanding of OverTD may differ depending on medical conditions, there may be underlying themes across conditions, so it is relevant to include studies relating to any conditions. The condition/s which a study focuses on will be noted. Study themes will be compared by conditions in analysis if the sample characteristics make this viable.