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Case series
A study reporting observations on a series of individuals, usually all receiving the same intervention, with no control group.

Case study
A study reporting observations on a single individual.  

Case-control study 
A study that compares people with a specific disease or outcome of interest (cases) to people from the same population without that disease or outcome (controls), and which seeks to find associations between the outcome and prior exposure to particular risk factors. This design is particularly useful where the outcome is rare and past exposure can be reliably measured. Case-control studies are usually retrospective, but not always.

Causal effect
An association between two characteristics that can be demonstrated to be due to cause and effect, i.e. a change in one causes the change in the other. Causality can be demonstrated by experimental studies such as controlled trials (for example, that an experimental intervention causes a reduction in mortality).  However, causality can often not be determined from an observational study.

CI
See Confidence interval

CINAHL (Cumulative Index of Nursing and Allied Health Literature)
Electronic database covering the major journals in nursing and allied health.Years of coverage: 1983 - present.

Clinical guideline
A systematically developed statement for practitioners and patients about appropriate health care for specific clinical circumstances.

Clinical trial
An experiment to compare the effects of two or more healthcare interventions.  Clinical trial is an umbrella term for a variety of designs of healthcare trials, including uncontrolled trials, controlled trials, and randomised controlled trials.  (Also called intervention study.)

Clinically significant
A result (e.g. a treatment effect) that is large enough to be of practical importance to patients and healthcare providers. This is not the same thing as statistically significant. Assessing clinical significance takes into account factors such as the size of a treatment effect, the severity of the condition being treated, the side effects of the treatment, and the cost. For instance, if the estimated effect of a treatment for acne was small but statistically significant, but the treatment was very expensive, and caused many of the treated patients to feel nauseous, this would not be a clinically significant result. Showing that a drug lowered the heart rate by an average of 1 beat per minute would also not be clinically significant.

Cluster randomised trial
A trial in which clusters of individuals (e.g. clinics, families, geographical areas), rather than individuals themselves, are randomised to different arms. In such studies, care should be taken to avoid unit of analysis errors.

Cochrane Collaboration
An international organisation that aims to help people make well informed decisions about health by preparing, maintaining and ensuring the accessibility of systematic reviews of the benefits and risks of healthcare interventions.

Cochrane Database of Systematic Reviews (CDSR)
One of the databases in The Cochrane Library. It brings together all the currently available Cochrane Reviews and Protocols for Cochrane Reviews. It is updated quarterly, and is available via the Internet and CD-ROM. See The Cochrane Library.

Cochrane Library (CLIB)
A collection of databases, published on CD-ROM and the Internet and updated quarterly, containing the Cochrane Database of Systematic Reviews, the Cochrane Central Register of Controlled Trials, the Database of Abstracts of Reviews of Effects, the Cochrane Methodology Register, the HTA Database, NHSEED, and information about The Cochrane Collaboration.

Cochrane Review
Cochrane Reviews are systematic summaries of evidence of the effects of healthcare interventions. They are intended to help people make practical decisions. 

Cohort study
An observational study in which a defined group of people (the cohort) is followed over time. The outcomes of people in subsets of this cohort are compared, to examine people who were exposed or not exposed (or exposed at different levels) to a particular intervention or other factor of interest. A prospective cohort study assembles participants and follows them into the future.  A retrospective (or historical) cohort study identifies subjects from past records and follows them from the time of those records to the present. Because subjects are not allocated by the investigator to different interventions or other exposures, adjusted analysis is usually required to minimise the influence of other factors (confounders).

Cointervention
The application of additional diagnostic or therapeutic procedures to people receiving a particular programme of treatment. In a controlled trial, members of either or both the experimental and the control groups might receive co-interventions.

Comorbidity
The presence of one or more diseases or conditions other than those of primary interest. In a study looking at treatment for one disease or condition, some of the individuals may have other diseases or conditions that could affect their outcomes.  (A co-morbidity may be a confounder.)

Comparison group
See control group.

Concealment of allocation
The process used to ensure that the person deciding to enter a participant into a randomised controlled trial does not know the comparison group into which that individual will be allocated. This is distinct from blinding, and is aimed at preventing selection bias. Some attempts at concealing allocation are more prone to manipulation than others, and the method of allocation concealment is used as an assessment of the quality of a trial. See also bias prevention. (Also called allocation concealment.)

Conference abstracts
Short summaries of presentations at conferences. May be published as proceedings.

Confidence interval (CI)
A measure of the uncertainty around the main finding of a statistical analysis.  Estimates of unknown quantities, such as the odds ratio comparing an experimental intervention with a control, are usually presented as a point estimate and a 95% confidence interval. This means that if someone were to keep repeating a study in other samples from the same population, 95% of the confidence intervals from those studies would contain the true value of the unknown quantity.  Alternatives to 95%, such as 90% and 99% confidence intervals, are sometimes used.  Wider intervals indicate lower precision; narrow intervals, greater precision.  (Also called CI.) 

Confidence limits
The upper and lower boundaries of a confidence interval.

Conflict of interest declaration [or Competing interests declaration]
A statement by a contributor to a report or review of personal, financial, or other interests that could have influenced someone.

Confounded comparison
A comparison between two treatment groups that will give a biased estimate of the effect of treatment due to the study design. For a comparison to be unconfounded, the two treatment groups must be treated identically apart from the randomised treatment. For instance, to estimate the effect of heparin in acute stroke, a trial of heparin alone versus placebo would provide an unconfounded comparison.  However, a trial of heparin alone versus aspirin alone provides a confounded comparison of the effect of heparin. (See also unconfounded comparison.)

Confounder
A factor that is associated with both an intervention (or exposure) and the outcome of interest. For example, if people in the experimental group of a controlled trial are younger than those in the control group, it will be difficult to decide whether a lower risk of death in one group is due to the intervention or the difference in ages. Age is then said to be a confounder, or a confounding variable.  Randomisation is used to minimise imbalances in confounding variables between experimental and control groups. Confounding is a major concern in non-randomised studies. See also adjusted analyses.

Contamination
[In a controlled trial:] The inadvertent application of the intervention being evaluated to people in the control group; or inadvertent failure to apply the intervention to people assigned to the intervention group. Fear of contamination is one motivation for performing a cluster randomised trial.

Context
The conditions and circumstances that are relevant to the application of an intervention, for example the setting (in hospital, at home, in the air); the time (working day, holiday, night-time); type of practice (primary, secondary, tertiary care; private practice, insurance practice, charity); whether routine or emergency.

Control

1. [In a controlled trial:] A participant in the arm that acts as a comparator for one or more experimental interventions. Controls may receive placebo, no treatment, standard treatment, or an active intervention, such as a standard drug.

2. [In a case-control study:] A person in the group without the disease or outcome of interest.

Control group

1. [In a controlled trial:] The arm that acts as a comparator for one or more experimental interventions. See also control. (Also called comparison group.)

2. [In a case-control study:] The group without the disease or outcome of interest. (Also called comparison group.)

Controlled before and after study
A non-randomised study design where a control population of similar characteristics and performance as the intervention group is identified. Data are collected before and after the intervention in both the control and intervention groups.

Controlled trial
A clinical trial that has a control group. Such trials are not necessarily randomised.

Conventional treatment
Whatever the standard or usual treatment is for a particular condition at that time.

Correlation
See association. (Positive correlation is the same as positive association, and negative correlation is the same as negative association.)

Cost-benefit analysis
An economic analysis that converts effects into the same monetary terms as the costs and compares them.

Cost-effectiveness analysis
An economic analysis that converts effects into health terms and describes the costs for some additional health gain (e.g. cost per additional stroke prevented).

Cost-utility analysis
n economic analysis that expresses effects as overall health improvement and describes how much it costs for some additional utility gain (e.g. cost per additional quality-adjusted life-year).

Cross-over trial
A type of clinical trial comparing two or more interventions in which the participants, upon completion of the course of one treatment, are switched to another. For example, for a comparison of treatments A and B, the participants are randomly allocated to receive them in either the order A, B or the order B, A.  Particularly appropriate for study of treatment options for relatively stable health problems. The time during which the first interventions  is taken is known as the first period, with the second intervention being taken during the second period.  

Cross-sectional study
A study measuring the distribution of some characteristic(s) in a population at a particular point in time. (Also called survey.)

Cumulative meta-analysis
A meta-analysis in which studies are added one at a time in a specified order (e.g. according to date of publication or quality) and the results are summarised as each new study is added. In a graph of a cumulative meta-analysis, each horizontal line represents the summary of the results as each study is added, rather than the results of a single study