Vivian A Welch, Jennifer Petkovic, Janet Jull, Lisa Hartling, Terry Klassen, Elizabeth Kristjansson, Jordi Pardo Pardo, Mark Petticrew, David J Stott, Denise Thomson, Erin Ueffing, Katrina Williams, Camilla Young, Peter Tugwell

Cite this chapter as: Welch VA, Petkovic J, Jull J, Hartling L, Klassen T, Kristjansson E, Pardo Pardo J, Petticrew M, Stott DJ, Thomson D, Ueffing E, Williams K, Young C, Tugwell P. Chapter 16: Equity and specific populations. In: Higgins JPT, Thomas J, Chandler J, Cumpston M, Li T, Page MJ, Welch VA (editors). Cochrane Handbook for Systematic Reviews of Interventions version 6.4 (updated August 2023). Cochrane, 2023. Available from www.training.cochrane.org/handbook.

16.1 Introduction to equity in systematic reviews

Health equity reflects a concern for social justice (Braveman 2006, Krieger 2008, Marmot et al 2008, Frieden 2011, Marmot et al 2012). When differences in health are avoidable, remediable and considered unjust and unfair, they are considered health inequalities (Whitehead 1992). Not all health differences are considered inequitable. For example, sickle cell disease is more common in some populations defined by ethnicity due to genetic differences and is not likely to be considered unfair. However, socio-economic differences in childhood asthma rates due to differential distribution of air pollutants would be considered an inequity. Reducing health inequities is considered an important public policy objective for social justice (i.e. moral grounds), social cohesion (for utilitarian reasons) and inter-generational solidarity (for sustainability).

We use the term ‘disadvantaged’ to denote disadvantage created by social, political and legal structures and processes (Welch et al 2015). Axes of potential disadvantage can be defined by the acronym PROGRESS-Plus (place of residence, race/ethnicity/culture/language, occupation, gender/sex, religion, education, socio-economic status and social capital) and ‘Plus’ refers to additional categories such as age, sexual orientation and disability which may influence opportunities for health of individuals and populations (O'Neill et al 2013). Other lists of characteristics may be helpful, depending on the intended audience of the review, such as the social determinants of health or SCRAP (sex, comorbidities, race, age and pathophysiology) (Dans et al 2008). The degree to which these factors are associated with disadvantage depends on time, place and interaction between the determinants (Lorenc et al 2013).

Review authors and decision makers increasingly recognize the importance of the impact of interventions on health equity. Some populations may not benefit from interventions to the same extent as others, which could lead to unintentional intervention-generated inequities (Lorenc et al 2013). Policy makers report that the lack of health equity considerations in systematic reviews limits their usefulness for decision making (Petticrew et al 2004).

Average results hide differences in effects between different populations. Therefore, review authors should consider not only what works on average, but also consider intervention impacts on health inequities. Systematic reviews may assess effects on health equity according to three types of interventions (Welch et al 2012):

1. interventions aimed at the general population, where it is important to understand the distribution of effects across one or more PROGRESS-Plus characteristics;
2. interventions focused on disadvantaged or at-risk populations in which there may not be equity outcomes but that may provide evidence about reducing inequities; and
3. interventions aimed at reducing social gradients across populations or among subgroups of the population.

Trials often exclude populations that are disadvantaged or those above or below a certain age. The exclusion of these populations may influence the applicability of results beyond the trial settings. Review authors should report on the characteristics of the populations according to relevant PROGRESS-Plus factors as well as whether there are population subgroups with a higher risk of the condition or problem or if there are differences in factors that influence access to care. Such factors include values, preferences, affordability and feasibility from the patient/public perspective and conscious or unconscious bias by practitioners. Wait times for total joint arthroplasty provide an example of practitioner bias and gender differences in access to care (Pederson and Armstrong 2015). These factors may vary according to context.

It is usually not feasible to assess all PROGRESS-Plus characteristics. Thus, in choosing characteristics to assess, review authors should consider the perspective of the intended beneficiaries of the interventions and the intended users of the evidence.

16.2 Formulation of the review

Five issues are important for formulating the review question: (i) defining health equity; (ii) hypotheses related to equity and logic models; (iii) appropriate study designs; (iv) appropriate outcomes; and (v) context.

16.2.1 Defining health equity

As health equity implies a judgement about fairness, the first step for review authors is to define which populations experience health inequity with respect to the condition/problem or intervention being assessed. For example, in a Cochrane Review of school meals, socio-economic status, gender and rurality were considered important factors associated with health inequity, but proxy measures were also used: baseline nutritional status was used as a proxy measure for socio-economic disadvantage (Kristjansson et al 2007). Justification for the use of proxies should be given, their use should be transparent and their limitations should be clearly reported.

Review authors may need to consider specific populations separately, either within a broader review or in a focused review, depending on the question and the intended recipients of the intervention. For example, it may be important to consider a separate review for indigenous peoples such as a review on family-centred interventions for indigenous childhood well-being (McCalman et al 2017). For interventions delivered to diverse populations, review authors should assess the primary studies for transparent reporting of participant demographics. It is also important to assess the need for sensitivity or subgroup analyses to explore potential differences in effects. Equity reviews can consider these differences across populations defined by one or more PROGRESS-Plus factors (e.g. migrants, linguistic minorities, homeless); however, they likely cannot address all PROGRESS-Plus factors. Thus, at the question formulation stage, review authors should explicitly consider which factors are most important and how they will be addressed in the methods of the review. Box 16.2.a provides information related to considerations for deciding whether there may be differences in the relevance or appropriateness of an intervention based on whether it is being implemented in low- and middle- and/or high-income countries. Box 16.2.b provides resources that may be helpful when planning systematic reviews of studies including children and youth.

Moreover, rather than using one category to describe people’s experiences, intersectionality illuminates the complex ways a person experiences discrimination simultaneously – across ageism, sexism, racism, and other forms of institutionalized discrimination (Hankivsky 2014).

For example, a Cochrane Review of school feeding for improving the physical and psychosocial health of disadvantaged students reported: “children were classified as ‘predominantly disadvantaged’ by …the following criteria: 1) Living in a rural area or village; 2) Living in an urban area and described as socio-economically disadvantaged (e.g. poor or low-income) or from poor areas (e.g. slums); 3) if statistics were presented showing that 30% or more of the children in the sample were underweight, or stunted (nutritionist judgement) or that the average weight, height, and Body Mass Index (BMI) were low (nutritionist judgement) and 4) studies were implicitly or explicitly aimed at disadvantaged children, and indicators of disadvantage were provided in the paper.”(Kristjansson et al 2007)

Box 16.2.a Low- and middle-income countries (LMICs)

Box 16.2.b Systematic reviews including children and youth

16.2.2 Logic models and theories of change to articulate hypotheses about equity

Analytic frameworks such as logic models, causal chains and funnels of attrition are increasingly being used in systematic reviews to identify key questions across the population, intervention, comparison group and outcomes (PICO) of interest (Chapter 2, Section 2.4). Funnel-of-attrition or equity-effectiveness frameworks explain why effect sizes decrease along the causal chain and allow for identification of the various factors such as coverage and uptake that may impact the implementation of an intervention (Tugwell et al 2008, White 2014). Logic models, which show the relationships between inputs and results, can help identify the key questions that are relevant to assessing effects on health equity by predicting likely differences in response, differences in baseline risk, applicability and also factors that may mediate effects. These factors and differences can guide the methods of the review. They can help scope the review question, identify eligibility criteria, focus the search strategy, design a process evaluation and consider relevance to policy and/or practice (Anderson et al 2011, O’Connor et al 2011). For example, a Cochrane Review of food supplementation for improving the physical and psychosocial health of socio-economically disadvantaged children included a logic model showing how socio-economic factors and family structure might modify effectiveness of supplementary feeding (Kristjansson et al 2015).

Theories of change provide a comprehensive description and illustration of how and why a desired change is expected to happen in a particular context (Mackinnon et al 2006, Kneale et al 2015). Pathways to change may be uncovered in the process of doing the review, therefore, theories of change may need to be updated and revised during the review process to incorporate discoveries about the processes and barriers and facilitators to implementation.

16.2.3 Appropriate study designs to assess equity

Eligible study designs should be chosen according to their fitness for purpose (Tugwell et al 2010), and the rationale should be clearly explained (see Chapter 3). Review authors need to consider whether non-randomized studies may provide relevant and meaningful evidence about the impact of the intervention in populations and settings that they consider important (Tugwell et al 2010). These different study designs need different assessment of potential bias (see Chapter 24).

16.2.4 Appropriate outcomes for equity

Outcomes need to be selected based on the stakeholder/user groups. A framework may be helpful in defining the relevant groups. For example, these could include the 9 ‘P’s: patients, practitioners, the public, policy makers, press, product makers (e.g. drug and devices manufacturers), payers (e.g. medical insurers), purchasers (e.g. employers, governments) and principal investigators (Concannon et al 2012, Rader et al 2014). Outcomes to be considered include benefits and harms (and their trade-off): mortality (general/condition specific), impact (symptoms, physical/emotional/social/spiritual function, quality of life, utility, inconvenience, financial burden) and intermediate/surrogate outcomes/biomarkers (Boers et al 2014). See Box 16.2.c for specific considerations for outcomes of importance for children and youth and older adults.

The relative importance of health and social outcomes may differ for populations who experience health inequity. For example, maternal employment, family income and education are important outcomes in a Cochrane Review of day care for preschool children of disadvantaged mothers (Zoritch et al 2000). These outcomes may be less important for mothers with higher socio-economic status. A similar analysis of relative importance could be applied to older adults with pension or other forms of social security, in contrast to those without. The importance of outcomes for different settings and populations needs to be rated when selecting outcomes for ‘Summary of findings’ tables (Chapter 14, Section 14.1.2). Context should be considered in rating importance of outcomes (Section 16.2.5). Additionally, inconvenience, burden (e.g. out-of-pocket costs, travel time) and stigma need to be considered as potential outcomes even if they are not commonly reported in primary studies since they may be of utmost importance to the intended recipients of the intervention.

Box 16.2.c Outcomes for child health or ageing

16.2.5 Context and equity

Review authors should consider the social, cultural and political contexts in which interventions are planned and implemented (Marmot et al 2008). Primary research on health and social interventions is conducted within particular temporal, cultural, geographical, political and organizational settings (Pope et al 2007), and these may influence intervention effectiveness (Hawe et al 2004).

‘Taking context into account’ means understanding the important aspects of context, how these may influence the intervention (e.g. implementation), and describing, stratifying and exploring the extent to which they influence outcomes (Lewin et al 2017). For example, for reviews including older adults, multimorbidity without integrated care, and overall declines in capacities are an important contextual issue. One aspect can be assessed with the number of prescribed medicines and therefore review authors may wish to report this indicator. Some tools have been developed to collect and extract data on context, including the Context and Implementation of Complex Interventions (CICI) framework (Pfadenhauer et al 2017).

Review authors may wish to assess and document whether research procedures in included studies meet international ethical standards, since populations experiencing health inequities may be vulnerable in research and need additional protections (Welch et al 2017a). Systematic reviews can reinforce ethical practices by identifying ethical concerns in included studies.

Variations in context between studies can be assessed qualitatively and/or quantitatively. Context may be described in different sections of the primary studies or in accompanying papers, reports, policies or historical documents; finding these descriptions may need expert knowledge (Noyes et al 2013). Thus, the full team and advisory board (if the review has one) or other key stakeholders should be involved in interpretation to ensure that the review is useful, relevant and applicable. For example, a Cochrane Review on environmental interventions to reduce the consumption of sugar-sweetened beverages reported: “the context in which included studies were done can therefore be essential for assessing the transferability and applicability of their results… We will therefore extract contextual data, using the categories defined by the CICI (Context and Implementation of Complex Interventions) framework.” (von Philipsborn et al 2016)

16.3 Identification of evidence

Searches for equity-focused reviews should follow the general guidance (Chapter 4), but should ensure there is enough coverage of populations of interest. Searches related to health equity are likely to address perspectives beyond the biomedical lens. Thus, potentially relevant studies may be found in a wider range of literature sources and may be unreliably categorized. This may influence the databases and search terms chosen. A Cochrane Review of interventions for promoting reintegration and reducing harmful behaviour and lifestyles in street-connected children and young people searched a broad range of websites and grey literature sources (Coren et al 2016).

16.3.1 Databases to consider

Non-health databases may be relevant if the outcomes of interest include, for example, labour productivity or educational, economic or social outcomes. The information retrieval guidance of the Campbell Collaboration is an excellent resource for searches related to social outcomes (Kugley et al 2017), while the Norwegian Satellite of the Effective Practice and Organisation of Care (EPOC) Group maintains a list of databases relevant for low- and middle-income countries (EPOC 2013). For example, a Cochrane EPOC review of strategies to increase the ownership and use of insecticide-treated nets to prevent malaria searched multiple databases in addition to MEDLINE and EMBASE, including: CINAHL, Web of Science, Dissertations and Theses, African Index Medicus, LILACS and WHOLIS (Augustincic Polec et al 2015).

16.3.2 Term selection and use of search filters for equity

Using standard search filters (i.e. those available in the search interface of a database) for equity-related content carries significant risks, as many of the words describing PROGRESS-Plus categories are not indexed in the major databases (MEDLINE/Pubmed added a new MeSH term, ‘health equity’, in 2016). Paediatric studies are also often poorly indexed. Authors of studies on children-specific conditions may fail to use paediatric terms explicitly in the title, abstract, or even within the manuscript. Therefore, when searching electronic databases, we recommend using a paediatric search filter (a combination of the subject headings, age limits [if available], and free text terms) rather than indexing or age limits alone. Searching for studies related to older people may consider available search filters for relevance (van de Glind et al 2012). When validated filters are available, their use will save time in building the search and in reducing the number of articles to screen. For example, validated search filters have been developed for sex and gender specific outcome data (Lorenzetti and Lin 2017) and for equity-focused studies (Prady et al 2018) may be helpful in designing searches. Additional filters can be found on the ISSG search filters resource (ISSG 2018).

16.3.3 Practical advice

Appropriate retrieval strategies vary, depending on the research question and the specific populations and settings included. Practical suggestions include the following.

• Use expert advice on planning and executing the search strategy, given the anticipated complexity of the searches (Chapter 4). Experts might know of unpublished, non-indexed or hard-to-locate evidence.
• Identify validated filters, considering sensitivity and specificity, and trying to correct known limitations. If the filter is not validated, consider carefully the risk of missing vital information.
• Look beyond traditional databases: small and specific databases addressing the research topic may be more relevant (Ogilvie et al 2005, Augustincic Polec et al 2015).
• Develop logic models to make explicit the decisions on the search strategy.
• Conduct iterative searches: language changes over time and varies by place.

16.4 Appraisal of evidence

For equity questions, baseline imbalance across PROGRESS-Plus factors may be important to assess by checking for poor randomization. Further, equity factors may be considered as potential confounders in non-randomized studies. Authors should document whether losses occurred differentially from specific populations defined by PROGRESS-Plus. Otherwise, the critical appraisal of evidence is similar to other reviews (discussed in Chapter 8 and Chapter 25).

16.5 Synthesis of evidence

Equity analysis involves three steps: first, identifying in the protocol which populations are likely to experience health inequity; second, assessing whether the intervention results in important improvement; and third, assessing whether the identified populations achieve the same improvement in both absolute and relative effects as other populations. Methods for assessing gradients of effects and gaps in absolute and relative effects are described by Evans et al (2001).

A Cochrane Review on culturally appropriate health education for type 2 diabetes mellitus in ethnic minority groups included equity considerations in the synthesis of the data: “we anticipated the need to stratify participants in age groups, as it can be an important effect modifier of outcomes; the effect of gender of participants, matched with gender of educators, were also analysed; … we tried to explore difference between different literacy subgroups, ability to speak language of the majority population and countries where the interventions take place; we stratified participants by ethnic groups.” Differences by age, gender and education were not explored because of insufficient data (Hawthorne et al 2008).

16.5.1 Subgroup analyses

For interventions provided to a broad population, equity may be considered through subgroup analyses across one or more PROGRESS-Plus factors, as pre-specified in the logic model and protocol (Chapter 2, Section 2.5.1).

Any subgroup analyses should be pre-specified and justified (Chapter 10, Section 10.11). In the process of doing the review, other important factors influencing outcomes may be uncovered. Authors should be open to this and all post-hoc decisions should be documented.

Meta-regression (Chapter 10, Section 10.11.4) may also be feasible to assess the role of explanatory variables such as population, context or process factors (Hollands et al 2015).

16.5.2 ‘Summary of findings’ tables

Authors may want to consider one of five methods to incorporate findings about health inequities in ‘Summary of findings’ tables (Welch et al 2017b):

1. include health equity as an outcome;
2. consider patient-important outcomes relevant to health equity;
3. present separate tables for populations who experience health inequity to highlight important differences in relative effectiveness;
4. create different rows within a single table to highlight differences in baseline risk for specific populations; and
5. assess indirectness of evidence for populations that are predefined as important who experience health inequity.

16.6 Interpretation of evidence

Interpretation of evidence for specific populations defined across PROGRESS-Plus should focus on those populations identified at the protocol stage as important recipients of the intervention. Interpretation should consider the questions: Are findings likely to be applicable in those populations, even if they did not make up a large proportion of the participant populations in included studies? Why or why not? This section should be transparent and rely on details in the ‘Summary of findings’ table for specific populations. Any subgroup analyses should be interpreted with caution (Chapter 10, Section 10.11.6). See Box 16.6.a for specific examples of issues with interpretation for reviews including older adults.

Box 16.6.a Issues with interpretation for reviews including older adults

16.7 Concluding remarks

We recommend that review authors explicitly consider the relevance of health equity to their review at the title and protocol stages using tools such as the Equity Checklist (Ueffing et al 2009), then design their methods accordingly to assess effects on health equity and/or discuss generalizability and applicability. Checklists for review authors are listed in Box 16.7.a.

Box 16.7.a Checklists for review authors

16.8 Chapter information

Authors: Vivian A Welch, Jennifer Petkovic, Janet Jull, Lisa Hartling, Terry Klassen, Elizabeth Kristjansson, Jordi Pardo Pardo, Mark Petticrew, David J Stott, Denise Thomson, Erin Ueffing, Katrina Williams, Camilla Young, Peter Tugwell

Acknowledgements: We would like to acknowledge Sari Tudiver and the Sex/Gender Methods Group whose work helped to inform this chapter and Ritu Sadana for useful comments. We acknowledge Simon Lewin for contributions to the equity concepts related to LMIC.

Funding: VAW holds an Early Researcher Award (2014–2019) from the Ontario Government. PT holds a Canada Research Chair in Health Equity (Tier 1), 2016–2024.

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