Reference

Glossary

Self-contained definitions of the research and statistics terms used across our publications. Each definition is written to be quotable on its own and carries a stable anchor link — cite any entry as /glossary#term-name. Definitions are licensed CC BY 4.0 like everything we publish.

#Cohort study

A cohort study follows a defined group of people over time, comparing outcomes between those exposed and not exposed to a factor of interest.

Because exposure is observed rather than assigned, cohort studies can establish temporal order but remain vulnerable to confounding.

#Confidence interval

A confidence interval is a range of values, calculated from sample data, that expresses the statistical uncertainty around an estimate; a 95% interval is produced by a procedure that captures the true value in 95% of repeated samples.

Wide intervals signal imprecise estimates. An interval that includes zero (or no difference) means the data are compatible with there being no effect — not proof that none exists.

Related: Meta-analysis

#Cross-sectional study

A cross-sectional study measures a population at a single point in time, providing a snapshot of conditions and associations but no information about temporal order.

Snapshots cannot distinguish whether A preceded B or the reverse, which limits causal interpretation.

Related: Cohort study

#Evidence hierarchy

An evidence hierarchy ranks study designs by their resistance to bias when answering causal questions, typically placing systematic reviews and randomised trials above observational designs and expert opinion.

Hierarchies apply to causal effectiveness questions specifically; for questions of mechanism, prevalence or lived experience, other designs are the appropriate tool.

#Excess mortality

Excess mortality is the difference between the number of deaths from all causes observed in a period and the number expected based on historical patterns, capturing the total mortality impact of a crisis.

See our full explainer: Excess Mortality: What It Measures and Why It Matters.

Related: Life expectancy

#Healthy life expectancy

Healthy life expectancy estimates the average number of years a person can expect to live in good health, combining mortality data with information on illness and disability.

The gap between life expectancy and healthy life expectancy indicates years lived with health limitations; it is central to debates about raising retirement ages.

Related: Life expectancy

#Life expectancy

Life expectancy at birth is the average number of years a newborn would live if current age-specific death rates remained constant throughout its life — a summary of present mortality conditions, not a forecast for any individual.

Because it is built from current death rates, period life expectancy can move sharply in crisis years and does not incorporate future medical progress.

#Meta-analysis

A meta-analysis statistically combines the results of multiple comparable studies into a single pooled estimate, increasing precision beyond what any individual study provides.

Pooling is only as good as the studies pooled: combining biased studies yields a precise biased answer. Meta-analyses are usually conducted within systematic reviews.

#Old-age dependency ratio

The old-age dependency ratio is the number of people aged 65 and over per 100 people of working age; conventions for “working age” (commonly 15–64 or 20–64) vary and materially change the figure.

The ratio counts ages, not actual work or dependency — a limitation that grows as working lives lengthen. See our population ageing primer.

Related: Total fertility rate (TFR)

#Peer review

Peer review is the evaluation of research by independent experts in the same field before publication, used by journals to assess validity, rigour and clarity.

Peer review is a quality filter, not a guarantee: reviewed papers can be wrong, and important work sometimes appears first as preprints.

Related: Preprint

#Preprint

A preprint is a research manuscript shared publicly before formal peer review, typically on servers such as arXiv, medRxiv or SSRN.

Preprints accelerate scientific communication but should be read as provisional; findings may change after review.

Related: Peer review

#Randomised controlled trial (RCT)

A randomised controlled trial assigns participants to an intervention or control group by chance, so the groups are comparable in expectation and outcome differences can be attributed to the intervention.

See our full primer: Randomised Controlled Trials in Public Policy.

#Replacement-level fertility

Replacement-level fertility is the average number of births per woman at which a population exactly replaces itself between generations without migration — about 2.1 in low-mortality countries.

The level exceeds 2.0 because slightly more boys than girls are born and some children do not survive to reproductive age; it is higher where child mortality is higher.

Related: Total fertility rate (TFR)

#Systematic review

A systematic review identifies, appraises and synthesises all studies meeting pre-specified criteria on a defined question, using an explicit, reproducible search method.

The pre-stated protocol is what distinguishes systematic reviews from narrative reviews, guarding against cherry-picking congenial studies.

#Total fertility rate (TFR)

The total fertility rate is the average number of children a woman would bear if she experienced the current year’s age-specific birth rates throughout her reproductive life.

TFR is a synthetic period measure: it can dip when births are postponed and later recover, without completed family sizes changing as much.

#Universal health coverage (UHC)

Universal health coverage, as defined by the World Health Organization, means all people have access to the health services they need — promotion, prevention, treatment, rehabilitation and palliative care — without suffering financial hardship.

UHC is measured along two dimensions: a service-coverage index and the incidence of catastrophic out-of-pocket health spending.

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