Health and population: statistics terms explained
Terms used in NHS, mortality and migration statistics, from incidence and prevalence to referral to treatment.
Incidence and prevalence
Incidence counts new cases arising in a period. Prevalence counts everyone who has the condition at a point or over a period, whether new or existing.
Incidence is the number (or rate) of new cases of a condition arising during a period, such as new diagnoses in a year. Prevalence is the number (or proportion) of people who have the condition at a given time, including long-standing cases.
Worked example
In a hypothetical town of 100,000, 500 people are living with a long-term condition on 1 January. During the year, 100 new cases are diagnosed and 50 people with the condition die.
- Annual incidence: 100 new cases, or 100 per 100,000.
- Prevalence at 1 January: 500, or 0.5%. By 31 December it is 550, or 0.55%.
How it trips up claims
- Prevalence can rise even if incidence falls, when better treatment means people live longer with a condition.
- More testing or better diagnosis can raise recorded incidence without a real change in how many people get the condition.
- Recorded prevalence (people on a GP register) is often lower than estimated prevalence, because some cases are undiagnosed.
The Fingertips service from the Office for Health Improvement and Disparities publishes many incidence and prevalence indicators for local areas in England.
Also searched as: incidence vs prevalence, prevalence rate, incidence rate, new cases
Related: Rate per 100,000 · Odds, risk, relative and absolute risk · Crude rate · What is the obesity rate in the UK? · What is the leading cause of death in the UK? · How many Covid deaths were there in the UK?
Cohort
A group of people who share a starting point, such as being born in the same year or starting school together, followed over time.
A cohort is a group defined by a shared event at the same time: born in the same year, entering school in the same September, or starting a benefit in the same month. Cohort statistics follow that group as it ages.
Period and cohort measures
Life expectancy shows the difference clearly. Period life expectancy uses death rates at each age in a single year, as if they never changed. Cohort life expectancy uses projected future improvements in death rates for people born in a given year. The ONS publishes both.
Worked example
Suppose a hypothetical period life expectancy at birth is 80 years. If death rates keep falling, a baby born that year may on average live longer, so the cohort figure could be 88. Both are valid; they answer different questions.
How it trips up claims
- "Average lifespan" claims often quote period life expectancy, which does not predict how long today's babies will live.
- Results for one school cohort, such as one year's GCSE entrants, can differ because of who is in that cohort, not only because of changes in teaching or exams.
- Comparing cohorts of different sizes calls for rates or percentages rather than counts.
Also searched as: birth cohort, period vs cohort life expectancy, cohort study, cohort analysis
Related: Age-standardised rate · Forecast and projection · Crude rate · What is the life expectancy in the UK? · How do I find life expectancy in my area? · Where can I find official GCSE results statistics?
Net migration
Long-term immigration minus long-term emigration over a period, usually a rolling year. It measures the net change in population from migration.
Net migration is the number of people who move to the UK for the long term (usually 12 months or more) minus the number who leave for the long term, over a given period. The ONS publishes long-term international migration estimates, usually for twelve-month periods ending June and December.
Worked example
If hypothetical long-term immigration in a year is 800,000 and emigration is 500,000, net migration is 300,000. If the next year immigration falls to 700,000 and emigration falls to 300,000, net migration rises to 400,000 even though fewer people arrived.
How it trips up claims
- Net migration is not the same as immigration. Check whether a claim refers to arrivals, departures or the difference.
- Small boat arrivals, asylum claims and visas granted are different measures from long-term net migration, and each has its own source.
- The latest ONS estimates are provisional and are often revised substantially as more data on whether people actually stayed become available.
- Figures cover a "year ending" period (a rolling twelve months), not the calendar year unless stated.
See the latest net migration figure.
Also searched as: net migration definition, long-term international migration, LTIM, immigration minus emigration
Related: Provisional figures · Revisions · Rolling average · Administrative data · What is net migration to the UK? · How many small boats crossed the Channel today? · How many asylum applications does the UK receive? · What is the population of the UK?
Referral to treatment (RTT)
The waiting time measure for planned hospital care in England: the time from referral to the start of consultant-led treatment. The NHS standard is 18 weeks.
Referral to treatment (RTT) measures how long patients in England wait from referral (usually by a GP) to the start of consultant-led treatment for planned, non-urgent care. The NHS Constitution sets an 18-week standard, with an operational target that 92% of patients on the waiting list should have been waiting less than 18 weeks. NHS England publishes the statistics monthly.
Pathways and patients
The headline "waiting list" counts incomplete pathways: waits that have started but not finished. One person can have more than one pathway if they are waiting for different treatments.
Worked example
A hypothetical trust has 10,000 open pathways; 6,500 have waited under 18 weeks. Performance is 65% against the 92% standard. If 1,000 pathways are removed after checking that patients no longer need treatment, the list falls without any extra treatment taking place.
How it trips up claims
- The number of pathways is not the same as the number of individual patients; NHS England also publishes an estimate of unique patients.
- RTT covers England only. Scotland, Wales and Northern Ireland use different measures (see comparing the four nations).
See the latest waiting list figure.
Also searched as: RTT, 18 week wait, NHS waiting list, referral to treatment time, incomplete pathways
Related: Headcount · Rolling average · England and Wales · How long is the NHS waiting list? · Is the NHS waiting list coming down? · How do I check NHS waiting times for my hospital? · What are NHS waiting times in Scotland, Wales and Northern Ireland?
Elective care
Planned, non-emergency hospital treatment such as hip replacements, cataract operations and many outpatient appointments, usually after a referral.
Elective care is planned treatment that is booked in advance rather than given in an emergency. It includes operations such as hip and knee replacements, diagnostic tests and consultant-led outpatient care. In England, waiting times for most elective care are measured under the referral to treatment (RTT) standard, which sets out that 92% of patients should be waiting less than 18 weeks.
Why it matters when checking a claim
- Waiting list and activity are different. The size of the waiting list measures people still waiting; activity measures how many were treated. Both can rise at once.
- "Elective" does not mean optional. It includes cancer treatment and other serious conditions.
- Interim targets. NHS England's planning guidance sets milestones short of the 92% standard; check which target a claim refers to.
- Each nation differs. Scotland, Wales and Northern Ireland measure elective waits in their own ways.
See how long is the NHS waiting list.
Also searched as: elective care meaning, planned care, elective surgery, elective waiting list
Related: Referral to treatment (RTT) · Waiting list pathways · Four-hour A&E standard · How long is the NHS waiting list? · Is the NHS waiting list coming down? · What are NHS waiting times in Scotland, Wales and Northern Ireland?
Four-hour A&E standard
The NHS standard that patients should be admitted, transferred or discharged within four hours of arriving at A&E. The constitutional standard is 95%.
The four-hour A&E standard measures the share of patients who are admitted, transferred or discharged within four hours of arrival at an accident and emergency department. The NHS Constitution standard in England is 95%; NHS England's planning guidance sets interim targets below that.
Why it matters when checking a claim
- Which type of A&E? Figures for all types include minor injury units and urgent treatment centres, where waits are shorter. Major emergency departments (type 1) usually perform worse. A claim should say which.
- 12-hour waits are measured two ways. From the decision to admit ("trolley waits") or from arrival. The from-arrival figure is much larger.
- Each nation publishes its own figures. Scotland, Wales and Northern Ireland use similar four-hour measures with their own targets and definitions.
See how long are A&E waiting times.
Also searched as: 4 hour target, A&E four hour wait, 4 hour A&E target, type 1 A&E, 12 hour trolley waits
Related: Ambulance response categories · Elective care · No criteria to reside · How long are A&E waiting times? · What are A&E waiting times in Northern Ireland? · What are ambulance response times in England?
Ambulance response categories
The four categories that ambulance calls in England are sorted into by urgency, each with its own response time standard.
In England, 999 calls to the ambulance service are sorted into four categories under the Ambulance Response Programme. Each has a standard set by NHS England:
- Category 1 (life-threatening, such as cardiac arrest): average 7 minutes, and 9 in 10 within 15 minutes.
- Category 2 (emergency, such as stroke or chest pain): average 18 minutes, and 9 in 10 within 40 minutes.
- Category 3 (urgent): 9 in 10 within 2 hours.
- Category 4 (less urgent): 9 in 10 within 3 hours.
Why it matters when checking a claim
- Mean or 90th centile? Both are published; they tell different stories about typical and long waits.
- Interim targets. Planning guidance has set interim Category 2 targets above 18 minutes.
- Other nations use different response models, so figures are not directly comparable.
See what are ambulance response times.
Also searched as: category 1 ambulance, category 2 ambulance response time, ambulance response times standards, Ambulance Response Programme
Related: Four-hour A&E standard · Mean · Median · What are ambulance response times in England? · How long are A&E waiting times?
Waiting list pathways
The unit the NHS in England uses to count its waiting list. One patient waiting for two different treatments counts as two pathways.
The NHS England referral to treatment waiting list counts pathways, not people. A pathway is one referral for one condition, from referral to the start of treatment. Someone referred for a knee problem and, separately, for a heart condition is on the list twice.
Why it matters when checking a claim
- "X million people waiting". The headline total is pathways. NHS England also publishes an estimate of the number of unique patients, which is lower. Check which figure is quoted.
- Validation. Trusts periodically check their lists and remove pathways where treatment is no longer needed, which can reduce the total without more treatment taking place. NHS England's notes explain such changes.
- England only. Scotland, Wales and Northern Ireland count waits differently; for example, NHS Wales also reports pathways.
See how long is the NHS waiting list and the NHS waiting list key figure.
Also searched as: RTT pathways, unique patients waiting list, waiting list pathways meaning, how many people are on the NHS waiting list
Related: Referral to treatment (RTT) · Elective care · Headcount · How long is the NHS waiting list? · Is the NHS waiting list coming down? · How long are NHS Wales waiting lists?
No criteria to reside
Hospital patients in England who are medically ready to leave but are still in a bed, often because care or support outside hospital is not yet arranged.
No criteria to reside describes patients who no longer need to be in an acute hospital bed, according to a clinical checklist, but have not yet left. NHS England publishes counts of these patients, and of how many were discharged each day, in its acute discharge situation report.
Why it matters when checking a claim
- It is broader than "bed blocking". Delays can be caused by hospital processes, transport, community health services or family decisions, as well as social care. The data record reasons for delay, so a claim blaming one cause can be checked against them.
- A daily snapshot. Figures are averages of daily counts over a month; individual days can differ.
- Coverage. They cover acute trusts in England; Scotland, Wales and Northern Ireland publish their own delayed discharge statistics.
See what is delayed discharge from hospital.
Also searched as: NCTR, delayed discharge, bed blocking, medically fit for discharge, delayed transfers of care
Related: Four-hour A&E standard · Elective care · What is delayed discharge, and how many hospital patients are affected? · How much do councils spend on adult social care? · How many people work in adult social care in England?
Healthy life expectancy
The average number of years a person can expect to live in good health, based on current death rates and people's own rating of their health.
Healthy life expectancy estimates how many years a person would live in "very good" or "good" general health, based on current mortality rates and how people describe their own health in surveys. The ONS publishes it for the UK, its nations and local areas, alongside life expectancy and disability-free life expectancy.
Why it matters when checking a claim
- A period measure. Like life expectancy, it describes current conditions, not a prediction for anyone born today.
- Self-reported. It relies on survey answers, which can change with expectations and survey methods, so small changes may not be meaningful.
- Gaps between areas. Differences between the most and least deprived areas are often larger for healthy life expectancy than for life expectancy. Local figures have wide confidence intervals.
- Multi-year periods. Estimates are for three-year periods, which smooths out year-to-year changes.
See what is the life expectancy in the UK.
Also searched as: HLE, healthy life expectancy uk, disability-free life expectancy, years in good health
Related: Confidence interval · Age-standardised rate · Cohort · What is the life expectancy in the UK? · How do I find life expectancy in my area? · Is the UK population ageing?
Asylum backlog
The number of asylum claims awaiting a decision. Usually means claims awaiting an initial Home Office decision, not appeals.
The asylum backlog usually refers to the number of asylum claims waiting for an initial decision from the Home Office. The Home Office publishes this figure each quarter in its immigration system statistics, with breakdowns by how long claimants have been waiting.
Why it matters when checking a claim
- Cases or people? Figures can be given for main applicants (cases) or for people including dependants. The second is larger.
- Initial decisions only. Appeals waiting at the immigration tribunal are counted separately by HM Courts and Tribunals Service and the Ministry of Justice. A falling Home Office backlog can coincide with a rising appeals backlog.
- Decisions are not removals. A refused claim may lead to an appeal, a further claim or a return, each counted elsewhere.
- Named backlogs. Government targets have sometimes applied to a defined group, such as a "legacy backlog" of older claims, rather than the whole queue.
See how many asylum applications does the UK receive.
Also searched as: asylum backlog uk, asylum claims awaiting decision, legacy backlog, asylum queue
Related: Asylum grant rate · Refugee status · Irregular migration (illegal entry routes) · How many asylum applications does the UK receive? · How many asylum seekers are in hotels? · How many people are deported or removed from the UK?
Asylum grant rate
The share of asylum decisions in a period that resulted in a grant of protection or other leave, usually calculated at the initial decision stage.
The asylum grant rate is the proportion of decisions on asylum claims that result in the person being allowed to stay. The Home Office's headline initial grant rate counts grants of refugee status, humanitarian protection and other forms of leave as a share of all initial decisions, excluding withdrawn claims.
Why it matters when checking a claim
- Initial or final? Some refused claimants win on appeal, so the share eventually granted can be higher than the initial rate. The Home Office publishes outcome analysis that follows claims through appeals.
- Nationality mix. Grant rates vary widely by nationality, so a change in the overall rate can reflect who is applying rather than a change in decision-making.
- Withdrawals. Large numbers of withdrawn claims can change the picture; check how they are treated.
See refugee status and how many asylum applications the UK receives.
Also searched as: asylum grant rate uk, initial grant rate, asylum success rate, asylum refusal rate
Related: Refugee status · Asylum backlog · Irregular migration (illegal entry routes) · How many asylum applications does the UK receive? · How many people are deported or removed from the UK?
Irregular migration (illegal entry routes)
People arriving in the UK without permission, such as by small boat or hidden in vehicles. Home Office statistics now use the term illegal entry routes.
Irregular migration means arriving in or staying in the UK without the required permission. Home Office statistics count people detected arriving by routes such as small boats across the English Channel, hidden in vehicles at ports, or at airports without valid documents. In 2026 the Home Office's immigration statistics adopted the term illegal entry routes for these figures; older releases used "irregular migration".
Why it matters when checking a claim
- Detected arrivals only. People who arrive undetected or overstay a visa are not in the arrival counts.
- Small boats are one route. Daily small boat figures are the most quoted, but quarterly tables cover all detected routes.
- Not the same as asylum claims. Many people arriving this way claim asylum; others do not, and many asylum claimants arrived legally.
- Different from net migration. Net migration estimates long-term moves by any route, including some people who arrived irregularly, so the two figures measure different things.
See how many small boats crossed the Channel and the small boat arrivals key figure.
Also searched as: illegal migration, illegal entry routes, small boat arrivals, irregular migration uk, irregular vs illegal migration
Related: Net migration · Asylum backlog · Asylum grant rate · Refugee status · How many small boats crossed the Channel today? · How many asylum applications does the UK receive? · How many people are deported or removed from the UK?
Refugee status
Protection granted to someone who has shown a well-founded fear of persecution in their own country under the 1951 Refugee Convention.
Refugee status is granted to a person who has shown that they have a well-founded fear of persecution in their home country for reasons of race, religion, nationality, political opinion or membership of a particular social group, as defined in the 1951 UN Refugee Convention. In the UK, an asylum seeker is someone who has applied and is waiting for a decision. If the claim is accepted, they are a refugee.
Why it matters when checking a claim
- Other forms of protection. Humanitarian protection is a separate status for people who do not meet the Convention definition but face a real risk of serious harm. Home Office grant figures often combine the two.
- Resettlement routes. People arriving through schemes for particular countries, such as Ukraine or Afghanistan, are counted in separate tables.
- Terms change. The length of leave and the route to settlement have changed over time; check the current GOV.UK guidance.
Also searched as: refugee meaning, asylum seeker vs refugee, humanitarian protection, refugee status uk
Related: Asylum grant rate · Asylum backlog · Irregular migration (illegal entry routes) · Net migration · How many asylum applications does the UK receive? · How many asylum seekers are in hotels?
Mid-year population estimates
The official annual estimates of how many people live in the UK and each area on 30 June, used as denominators for most per-head figures.
Mid-year population estimates are the official estimates of the usually resident population on 30 June each year. The ONS produces them for England and Wales and the UK, National Records of Scotland for Scotland and NISRA for Northern Ireland. They start from the census and roll forward using births, deaths and migration.
Why it matters when checking a claim
- They sit under most rates. Figures per head or per 100,000 use these estimates, so revisions to population change those rates too.
- Migration drives revisions. Updated net migration estimates can lead to sizeable changes to past population figures.
- Estimates, not counts. Between censuses, uncertainty grows, particularly for local areas with many students or migrants.
- Projections are different. Population projections show what would happen if assumed trends continued.
See what is the population of the UK and the population key figure.
Also searched as: MYE, population estimates uk, mid-year estimates, ONS population estimate
Related: Census · Net migration · Per capita (per head) · Dependency ratio · Revisions · What is the population of the UK? · Why is the UK population growing? · What are the UK population projections?
Dependency ratio
The number of people of pension age, or of children and pensioners, compared with the number of people of working age.
A dependency ratio compares the number of people in age groups less likely to be working with the number of working age. The old-age dependency ratio used by the ONS is the number of people over State Pension age for every 1,000 people aged between 16 and State Pension age. A total dependency ratio also includes children.
Why it matters when checking a claim
- Definitions differ. UK figures often use State Pension age, which is rising, while international bodies usually use fixed ages such as 65. The same population can give different ratios.
- Age is not activity. The ratio counts ages, not whether people actually work. Employment among older people and inactivity among working-age people both matter.
- Projections. Future ratios come from population projections, which depend on assumptions about migration, births and life expectancy.
See is the UK population ageing.
Also searched as: old-age dependency ratio, dependency ratio uk, OADR, ageing population ratio
Related: Mid-year population estimates · Economic inactivity · Cohort · Is the UK population ageing? · What are the UK population projections? · How many people get the State Pension?