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Useful terms

Active smoker

An active smoker includes anyone who is a current smoker (smokes at the present time), ever smoker, ex- or former smoker (someone who has previously smoked). The reviews included the smoking of cigarettes, cigars, pipes and rolled tobacco only. Use of other smoking products such as the smoking of shisha, electronic cigarettes and smokeless tobacco products were not included in the analysis.

Bronchial responsiveness

Bronchial hyper responsiveness is where the airways of the lungs (bronchi) are overly sensitive to factors that may irritate the lungs. It is a characteristic seen in chronic obstructive pulmonary disease (COPD) and asthma.

Bronchitis / Bronchiolitis

Acute bronchitis is a short-term infection of the airways affecting between 30-50 people in every 1,000 per year.
Bronchiolitis is an infection of the lower airways affecting babies and infants under 2 years of age. It is the most common cause of admission to hospital for babies under the age of 1 year.

Case control study / Nested case-control study

A case-control study is a type of study design used widely, often in epidemiology. It is a type of observational study in which two existing groups with different outcomes are identified and compared on the basis of a risk factor. Case-control studies are often used to identify factors that may contribute to a medical condition by comparing subjects who have that condition (the "cases") with patients who do not have the condition but are otherwise similar (the "controls").

A nested case-control study is a special type of case-control study in which people who are being studied (the “cases”) are taken from the same population of people as the controls to whom they are compared. These studies are sometimes called case-control studies nested in a cohort or case-cohort studies.

Confidence intervals

A confidence interval is a statistical range with a specified probability that a given parameter lies within the range. In the case of 95% confidence intervals the probability is 95%. Certain factors may affect the confidence interval size including size of sample, level of confidence and population variability. A larger sample size normally will lead to a better estimate of the population parameter.


An ever-smoker is a person who has smoked at least one hundred cigarettes/cigars during the course of his or her lifetime.

Functional residual capacity

Functional Residual Capacity (FRC) is the amount of air that is in the lungs when a person has breathed out normally.

Longitudinal studies (cohort studies)

A longitudinal cohort study is an observational research method in which data is gathered for the same subjects repeatedly over a period of time. Longitudinal research projects can extend over years or even decades. Longitudinal studies track the same people, and therefore the differences observed in those people are less likely to be the result of cultural differences across generations.

Types of longitudinal cohort studies include:

Prospective cohort study

A prospective study is a longitudinal study that looks forward in time. The population selected is healthy at the beginning of the study. Some of the members of the group share a particular characteristic, such as cigarette smoking. The researcher follows the population group over a period of time, noting the rate at which a condition, such as lung cancer, occurs in the smokers and in the nonsmokers. A prospective study may involve many variables or only two; it may seek to demonstrate a relationship that is an association or one that is causal.

Retrospective cohort study

A retrospective study is a longitudinal study that takes a look back at events that already have taken place using already collected (historical) data on exposures. The researcher collects data from past records and does not follow patients up.

Lung volume

Lung volume refers to the amount of air in the lungs at any one time. The average total lung capacity of an adult human male is about 6 litres of air, but only a small amount of this capacity is used during normal breathing.

Forced expiratory volume

Forced expiratory volume (FEV) is the amount of air that can forcibly be blown out in a set time period after a full breath in. This time period is usually ½ or one second.

Forced vital capacity

Forced vital capacity (FVC) is the amount of air that can forcibly be blown out after a full breath in.

Forest plot

A forest plot is a graphical way of presenting data that illustrates the relative strength of treatment effects in scientific studies addressing the same question. It was developed for use in medical research as a means of graphically representing a meta-analysis.
In a forest plot:

Funnel plot

A funnel plot is a useful graph designed to check the existence of publication bias in systematic reviews and meta-analyses. It assumes that the largest studies will be near the average, and small studies will be spread on both sides of the average. Variation from this assumption can indicate publication bias.


Incidence is a measure of the risk of developing a new condition within a specified period of time. Although sometimes loosely expressed simply as the number of new cases during a specific time period, it is better expressed as a proportion or a rate.


Mortality rate is a measure of the number of deaths in a population, scaled to the size of that population, per unit of time.


This is a mathematical technique that combines the results of individual studies to arrive at one overall measure of the effect.

Minute ventilation

Minute ventilation is the amount of air breathed in by an individual in one minute.

Newcastle Ottawa scale

The Newcastle-Ottawa Scale (NOS) was developed to assess the quality of nonrandomised studies used in a systematic review. A “star system” has been developed in which a study is judged on three broad perspectives: the selection of the study groups; the comparability of the groups; and the ascertainment of either the exposure or outcome of interest for case-control or cohort studies respectively.


P is short for probability. It is the statistical probability of the occurrence of a given finding by chance alone in comparison with the known distribution of possible findings, considering the kinds of data, the technique of analysis and the number of observations. The p-value may be noted as a decimal: P <.05 means that the likelihood that the phenomena tested occurred by chance alone is less than 5%. The lower the p-value, the less likely the finding would occur by chance alone.


Pack-years is a way to measure the amount a person has smoked over a long period of time. It is calculated by multiplying the number of packs of cigarettes smoked per day by the number of years the person has smoked. For example, 1 pack year is equal to smoking 1 pack per day for 1 year, or 2 packs per day for half a year.

Passive smoking

Passive smoking is when someone is in contact with second-hand smoke or environmental tobacco smoke from any source, both in the home or workplace. It doesn’t, however, include passive smoke exposure relating to cooking fuels. It occurs when tobacco smoke is in any environment, causing people within that environment to breathe it in.

Peak expiratory flow

Peak expiratory flow (PEF) is the maximal flow (or speed) of air breathed out while forcefully breathing out after a full breath in.

Publication bias

Publication bias occurs if scientific studies with negative or null results fail to get published. This can happen due to bias in submitting, reviewing, accepting, publishing or aggregating scientific literature that fails to show positive results on a particular topic. Publication bias can make scientific literature unrepresentative of the actual research studies. This can give the reader a false impression about the beneficial effects of a particular treatment or intervention and can influence clinical decision making.

Relative risk ratio

Relative risk (RR) is the ratio of the probability of an event occurring (for example, developing a disease, being injured) in an exposed group, to the probability of the event occurring in a comparison, non-exposed group. Relative risk includes two important features: a comparison of risk between two "exposures" puts risks in context, and, two, "exposure" is ensured by having proper denominators for each group representing the exposure.
Pooled relative risk ratio - When data from multiple studies are combined to give one overall result, for example when completing a meta-analysis.

Respiratory rate

Respiratory rate is the number of breaths taken in a specified period of time.

Systematic review

A systematic review is a bringing together of all the medical research on a particular subject. It uses thorough methods to identify, appraise, select and synthesise all high quality research evidence relevant to that question. Typically it pulls together the results from all the studies identified through statistical techniques, such as meta-analysis, to give one overall result. Results from systematic reviews can be displayed in a forest plot.

Tidal volume

Tidal volume is the amount of air breathed in and out during normal breathing.