Understanding the Terms:
Absolute Risk: Absolute risk is your risk of developing a disease over a time-period. The same absolute risk can be expressed in different ways. For example, say you have a 1 in 10 risk of developing a certain disease in your life. This can also be expressed as a '10% risk', or 'a 0.1 risk' - depending if you use percentages or decimals.
Before and After Study: a "Before and After Study" is one in which characteristics of a population or a group of individuals are compared before and after a particular event or intervention (such as the introduction of a new treatment) to see what the effects of the event or intervention have been.
Blinding: Blinding is when individuals are not told what treatment they have received or in some cases, what their outcome has been, to avoid them being influenced by this knowledge. The person who is 'blinded' could be either the person who is being treated or the researcher assessing the effect of the treatment (single blind), or both of these people (double blind).
Case-control Study: Case-control studies are those which are often used to identify risk factors for a medical condition. This type of study compares a group of patients who have that condition with a group of patients that do not and looks back in time to see how the characteristics of the two groups differ.
Case Crossover Study: Case crossover studies are those which are used to look at the side effects of factors that are thought to increase risk of a particular outcome in the short term. For example, this type of study might be used to look at the effects of changes in air pollution levels on the short-term risk of asthma attacks. In this type of study, individuals who have had the outcome of interest are identified and they act as their own control. The presence or absence of the risk factor is assessed for the period immediately before the individual experienced the outcome, and compared with the presence or absence of the risk factor in a control time period, when the individual did not experience the outcome. If there is a link between the risk factor and the outcome, then it would be expected to be present in the period just before the outcome more often than in the control period.
Case Series: A descriptive study of a group of people, usually receiving the same treatment or with the same disease. In this type of study, it is possible to describe characteristics or outcomes in a particular group of people, but not to determine how this compares with people who are not treated in the same way or do not have this condition.
Clinical Practice Guidelines: Clinical practice guidelines are systematically developed statements to assist practitioner and patient make decisions about appropriate health care for specific clinical circumstances.
Cluster Randomised Controlled Trial: A cluster randomised controlled trial is one in which people are randomised in groups (or "clusters") rather than on an individual basis. Types of "clusters" that could be used include schools, neighbourhoods, or GP surgeries.
Cohort Study: This is a study which identifies a group of people and follows them over a period of time to see how their exposures affect their outcomes. This type of study is normally used to look at the effect of suspected risk factors that cannot be controlled experimentally on outcomes, for example the effect of smoking on lung cancer.
Confounding Factor/Confounder: This is something that can distort the true relationship between two (or more) characteristics. When it is not taken into account, false conclusions can be drawn about associations. An example is to conclude that if people who carry a lighter are more likely to develop lung cancer, it is because carrying a lighter causes lung cancer. In fact, smoking is a confounder here. People who carry a lighter are more likely to be smokers and smokers are more likely to develop lung cancer.
Control Group: A 'control' group (cells, individuals, centres) serves as a basis of comparison in a study. In this 'group', no experimental stimulus is received.
Cross Sectional Study: These are epidemiological studies which describe characteristics of populations. They are 'cross sectional' because data is collected at one point in time and then the relationships between characteristics are considered. Importantly, because such a study doesn't look at time trends, it can't establish what causes what.
Diagnostic study: This is a type of study that tests a new diagnostic method to see if it is as good as the 'gold standard' method' of diagnosing a disease. The diagnostic method may be used when people are suspected of having a disease because of signs and symptoms, or it may be used to try and detect people before they have any symptoms, a screening method.
Ecological Study: In ecological studies the unit of observation is the population or community. Common types of ecological study are geographical comparisons, time trend analysis or studies of migration.
Epidemiology: Epidemiology is the study of factors affecting the health and illness of populations.
Experiment: An experiment is any study where the conditions are under the direct control of the researcher. Usually this involves giving a group of people an intervention that would not have occurred naturally. Such studies are often used to test the effects of a treatment in people and usually involve comparison with a group who do not get the treatment.
Genome-wide Association Study: This study looks across the entire genetic sequence (genome) to identify particular variations in the genetic sequence that are more common in people with a particular characteristic or condition, and therefore might be involved in producing the characteristic or condition.
Levels of Evidence: A hierarchical categorization of different types of clinical evidence partly based on study type that ranks evidence according to its ability to avoid the various biases in medical research. Several schemes exist that are specific to the question posed in the research. The general ranking of studies for a question starts with those that provide best evidence that a result is true. For example, from high level to low level evidence: systematic reviews, single randomised controlled trials, controlled trials without randomisation, prospective cohort studies, case-control studies, cross sectional studies, case series, single case reports. The expert opinions of respected authorities, based on clinical experience, descriptive studies, physiology, bench research or first principles are often thought of as the lowest level evidence. Although there are different systems, some of which take into account other aspects of quality including the directness of the research, the levels are designed to guide users of clinical research information as to which studies are likely to be most valid.
Longitudinal Study: A longitudinal study is one that studies a group of people over a period of time.
Meta-analysis: Mathematical technique to combine the results from individual studies to arrive at one overall measure of treatment effect.
Narrative Review: A narrative review discusses and summarises the literature on a particular topic, without generating any pooled summary figures using meta-analysis. This type of review usually aims to give a comprehensive overview of a topic, rather than address a specific question, such as how effective a treatment is for a particular condition. Narrative reviews often do not report on how they searched for literature or how they decided which studies were relevant to include, and therefore are often not classified as systematic reviews.
"Nested" Case-control Study: A "nested" case-control study is a special type of case control study design where "cases" of a disease are drawn for the same cohort (population of people) as the controls to which they are compared. They are sometimes referred to as a case-control study nested in a cohort or a case-cohort study. The collection of data on the cases and controls is defined before the study begins. Compared with a simple case-control study, the nested case-control study can reduce 'recall bias' and temporal ambiguity, and compared with cohort study, it can reduce cost and save time. Incidence rates can sometimes be estimated from a nested case-control study cohort whereas they cannot from a simple case-control study (as the denominator is not known).
Non-randomised Study: In the context of study design, this means that participants were not allocated randomly to receiving an intervention or not.
Observational Study: An observational study is one where the researchers have no control over exposures and instead observe what happens to groups of people.
Open Label: In the context of study design, this means that investigators and participants in a randomised controlled trial were aware of what treatment was being given and received, i.e. the study is not blinded.
Peer Review: Peer review is giving a scientific paper to one or more experts in that field of research to ask whether they think it is of good enough quality to be published in a scientific journal. Studies that are not of sufficient quality will not be published if their faults are not corrected. Journals that use peer review are thought to be of better quality than those which do not.
Phase I trials: This is the first time that the drug is tested in humans. These are usually quite small studies and aim to test the drug's safety and suitability for use in humans rather than its actual effectiveness. These usually involve between 20-100 healthy volunteers, although it may involve people with the condition the drug is aimed at treating. To test the drug's safe dosage range, very small doses are given and gradually increased until the levels suitable for use in humans are found. These studies also see how the drug behaves in the body, examining how it is absorbed, where it is distributed, how it leaves the body and how long it takes to do this.
Phase II Trials: During this phase, the drug's effectiveness in treating the targeted disease in humans is examined for the first time and more is learnt about appropriate dosage levels. This stage usually involves 200-400 volunteers who have the disease or condition that the drug is designed to treat. The drug's effectiveness is examined and more safety testing and monitoring of the drug's side effects are carried out.
Phase III Trials: In this phase, the effectiveness and safety of the drug undergoes a rigorous examination in large, carefully controlled trial to see how well it works and how safe it is. The drug is tested in a much larger sample of people with the disease/condition than before, with some trials including thousands of volunteers. Participants are also followed up for longer than in previous phases, sometimes several years. These controlled tests usually compare the new drug's effectiveness against either existing drugs or a placebo. These trials are designed to give the drug as unbiased a test as possible, ensuring that the results are an accurate view of its benefits and risks. The large numbers of participants and the extended period of follow-up give a more reliable indication of whether the drug will work and allows rarer or longer-term side effects to be identified.
Pre-clinical Evaluations: These are in vitro (for example in cell cultures) and in vivo laboratory animal tests on drugs in development, carried out to ensure that they are safe and effective before they go on to be tested in humans (clinical studies).
Prevalence: Prevalence is how common a particular characteristic (for example a disease) is in a specific group of people or a specific population. Prevalence is usually assessed using a cross sectional study.
Prospective Observational Study: A study which identifies a group of people and follows them over a period of time to see how their exposures affect their outcomes. This type of study is normally used to look at the effect of suspected risk factors that cannot be controlled experimentally on outcomes, for example the effect of smoking on lung cancer.
Prospective Study: A prospective study asks a specific study question, usually about how a specific exposure(s) affects a specific outcome(s), recruits appropriate participants, and looks at the specific exposures and outcomes of interest in these people over the following months or years.
Publication Bias: Publication bias arises from the tendency for researchers and editors to handle experimental results that are positive differently from results that are negative or inconclusive. It is especially important to detect this effect in studies that attempt to pool the results of several trials.
Qualitative Research: Qualitative research uses individual in-depth interviews, focus groups or questionnaires to collect, analyse and interpret data by observing what people do and say. It reports on the meanings, concepts, definitions, characteristics, metaphors, symbols and descriptions of things. It is more subjective than quantitative research and is often exploratory and open-ended. Small numbers of people are interviewed in-depth and/or a relatively small number of focus groups are conducted.
Quantitative Research: Quantitative research uses statistical methods to count and measure outcomes from a study. The outcomes are usually objective and predetermined. A large number of participants are usually involved to ensure that the results are statistically significant.
Randomised Controlled Trial (ReT): This is a study where people are allocated randomly to receiving a particular intervention or not (this could be two different treatments or one treatment and a placebo). This is the best type of study design to determine whether a treatment is effective.
Randomised Crossover Trial: A study where people receive all of the treatments and controls being tested, and are assigned to receive these treatments in a random order. This mean that people receive one treatment and the effect of this treatment is measured, and then they 'cross over' into the other treatment group, and the effect of the second treatment (or control) is measured.
Relative Risk: Relative risk is used to compare the risk in two different groups of people. All sorts of groups are compared to others in medical research to see if belonging to a particular group increases or decreases your risk of developing certain diseases. This type of risk is often expressed as a percentage increase or decrease, for example, 'a 20%increase in risk' of treatment A compared to treatment B. If the relative risk is 300%, it may also be expressed as 'a threefold increase'.
Retrospective Study: A retrospective study relies on data on exposures and/or outcomes that have already been collected (through medical records or as part of another study). Data used in this way may not be as reliable as data collected prospectively, as it relies on the accuracy of records made at the time, and on people's recall of events in the past, which can be inaccurate (referred to as recall bias).
Secondary Analysis: A secondary analysis is when researchers revisit data that was collected for a different reason and analyse it again to answer a new research question. Doing this can sometimes increase errors.
Selection Bias: Selection bias is a distortion of evidence or data that arises from the way that the data is collected.
Statistical Significance: Statistical significance means that the results are not likely to have occurred by chance alone. In such cases, we can be more confident then that we are observing a 'true' result.
Systematic Review: A synthesis of the medical research on a particular subject. It uses thorough methods to search for and include all or as much as possible of the research on the topic. Only relevant studies, usually of a certain minimum quality, are included.
Time Trend Studies: Time trend studies are epidemiological studies which describe characteristics of a population over a period of time. They look at trends at the population level, rather than at the level of individual people, through taking repeated cross sectional samples.
Tissue Engineering: Tissue engineering is an interdisciplinary field that applies the principles of engineering and biological sciences toward the development of functional substitutes for damaged tissue.
Twin Studies: Twin studies rely on comparing the phenotypes (observable physical traits) of monozygotic twins (genetically identical) and dizygotic (non-identical) twin pairs. The difference in correlation between phenotypes in the identical twins and the correlation in phenotypes in the fraternal twins estimate the genetic contribution to variations in phenotype: the within-twin correlation.