BLIND or BLINDED. Masked. Unaware. The term may be modified according to the purpose of the blinding. For example, clinicians or patients can be blind to the treatments that patients are receiving and observers can be blind to each other´s assessments, making their observations uninfluenced by one another (see also DOUBLE BLIND). To avoid confusion, the term MASKED is preferred in studies in which vision loss is an outcome of interest.

CASE-CONTROL STUDY (CASE-REFERENT OR CASE-COMPARISON STUDY). Study generally used to test possible causes of a disease or disorder, in which individuals who have a designated disorder are compared with individuals who do not have the disorder with respect to previous or current exposure to a putative causal factor. For example, persons with hepatic cancer (cases) are compared with persons without hepatic cancer (controls) and history of hepatitis B is determined for the two groups. A CASE-CONTROL STUDY is often referred to as a RETROSPECTIVE STUDY (even if patients are recruited prospectively) because the logic of the design leads from effect to cause.

CASE SERIES. A series of patients with a defined disorder. The term is usually used to describe a study reporting on a consecutive collection of patients treated in a similar manner, without a concurrent control group. For example, a surgeon might describe the characteristics of and outcomes for 100 consecutive patients with cerebral ischemia who received a revascularization procedure. See also CONSECUTIVE SAMPLE.

COHORT. A group of persons with a common characteristics or set of characteristics. Typically, the group is followed for a specified period to determine the incidence of a disorder or complications of an established disorder (that is prognosis), as in COHORT ANALYTIC STUDY (prospective study) (see also INCEPTION COHORT).

COHORT ANALYTIC STUDY. Prospective investigation of the factors that might cause a disorder in which a cohort of individuals who do not have evidence of an outcome of interest but who are exposed to the putative cause are compared with a concurrent cohort who are also free of the outcome but not exposed to the putative cause. Both cohorts are then followed to compare the incidence of the outcome of interest.

CONFOUNDER, CONFOUNDING VARIABLE. A factor that distorts the true relationship of the study variables of central interest by virtue of being related to the outcome of interest but extraneous to the study question and unequallly distributes among the groups being compared. For example, age might confound a study of the effect of a toxin on longevity if individuals exposed to the toxin were older thas those not exposed.

CONSECUTIVE SAMPLE. Sample in which the units are chosen on a strict "first come, first chosen" basis. All individuals who are eligible should be included as they are seen.

CONVENIENCE SAMPLE. Individuals or groups selected at the convenience of the investigator or primarily because they were available at a convenient time or place.

COST-BENEFIT ANALYSIS. An economic evaluation in which alternative programs, services, or interventions are compared in terms of the cost per unit of clinical effect (form example, cost per life saved, cost per millimeter of mercury of blood pressure lowered, or cost per quality-adjusted life-year gained). The last form of measuring outcomes (and equivalents such as "healthy days of life gained") gives rise to what is also referred to as COST-UTILITY ANALYSIS.

COST-UTILITY ANALYSIS. See also COST-EFFECTIVENESS ANALYSIS.

CRITERION STANDARD. Preferred term to "gold standard". A method having established or widely accepted accuracy for determining a diagnosis, providing a standard to which a new screening or diagnostic test can be compared. The method need not be a single or simple procedure, but could include follow-up of patients to observe the evolution of their conditions or the consensus of an expert panel of clinicians. CRITERION STANDARD can also be used in studies of the quality of care to indicate a level of performance, agreed to by experts or peers, to which the performance of individual practitioners or institutions can be compared.

CROSSOVER TRIAL. A method of comparing two or more treatments or interventions in which subjects or patients, on completion of the course of one treatment, are switched to another. Typically, allocation to the first treatment is by random process. Participants’ performance in one period is used to judge their performance in others, usuallly reducing variability. See also BEFORE-AFTER TRIAL.

DATA SET. Raw data gathered by investigators.

DOUBLE-BLIND OR DOUBLE MASK. (1) Neither subject nor study staff (those responsible for patient treatment and data collection) are aware of the group or internention to which the subject has been assingned. (2) Any condition in which two different groups of persons are purposely dennied access to information in order to keep that information from influencing some measurement, observation, or process.

ECONOMIC EVALUATION. Comparative analysis of alternative courses of action in terms of both their costs and consecuences.

END POINT. See OUTCOMES

GOLD STANDARD. See CRITERION STANDARD.

INCEPTION COHORT. A designated group of persons, assembled at a common time early in the development of a specific clinical disorder (for example, at the time of first exposure to the putative cause or at the time of initial diagnosis), who are followed thereafter (see also COHORT)

MASKED. See BLIND

MATCHING. The deliberate process of making a study group and a comparison group comparable with respect to factors that are extraneous to the purpose of the investigation but that may interfere with the interpretation of the study’s findings (for example, in case-control studies, individual cases might be matched or paired with a specific control on the basis of comparable age, gender, clinical features or a combination).

NONRANDOMIZED CONTROL TRIAL. Experiment in which assignment of patients to the intervention groups is at the convenience of the investigator or according to a preset plan that does not conform to the definition or RANDOM.. See also RANDOMIZED CONTROL TRIAL.

OUTCOMES. All possible changes in health status that may occur in following subjects or that may stem from exposure to a causal facter or from preventive or therapeutic interventions. The narrower term END POINTS refers to health events that lead to completion or termination of follow-up of an individual in a trial or cohort study, for example, death or major morbidity, particularly related ot the study question.

PRIMARY CARE. Medical care provided by the clinician of first contact for the patient. Typically, the primary care physician is a general practitioner, family practitioner, primary care internist, or primary care pediatrician. Primary care may also be administered by health professionals other than physicians, notably, specially trained nurses (nurse practitioners) and paramedics. Usually, a general practitioner , family practitioner, nurse practitioner, or paramedic provides only primary care services, but a person with specilaty qualifications may provide primary care, alone or in combination with referral services (see also REFERRED CARE). Thus, it is the nature of the contact (first compared with referred) that determines the care designation rather than the qualifications of the pracititioner.

PRIMARY CARE CENTER, PRIMARY CARE SETTING. Medical care facility that offers first-contact health care only. Patients requiring specialized medical care are referred elsewhere. Some primary care centers provide a mixture of primary and referred care. Thus it is the nature of the care provided (first contact) rather than the setting per se that distinguishes primary from more advanced levels of care. See also PRIMARY CARE, REFERRED CARE, TERTIARY CARE CENTER.

PROSPECTIVE STUDY. See COHORT and COHORT ANALYTIC STUDY.

RANDOM. Governed by a formal chance process in which the occurrence of previous events is of no value in predicting future events. The probability of assignment of, for example, a given subject to a specified treatment group is fixed and constant (typically 0.5) but the subject’s actual assignment cannot be known until it occurs.

RANDOM SAMPLE. A sample derived by selecting sampling units (for example, individual patients) such that each unit has an independent and fixed (generally equal) chance of selction. Whether a given unit is selected is determined by chance (for example, by a table of randomly ordered numbers).

RANDOMIZATION, RANDOM ALLOCATION. Allocation of individuals to groups by chance, usually done with the aid of a table of random numbers. Not to be confused with systematic allocation (for example, on even and odd days of the month) or allocation at the convenience or discretion of the investigator.

RANDOMIZED TRIAL (RANDOMIZED CONTROL[LED] TRIAL, RANDOMIZED CLINICAL TRIAL, RCT). Experiment in which individuals are randomly allocated to receive or not receive an experimental preventive, threapeutic, or diagnostic procedure and then followed to determine the effect of the intervention.

REFERRED CARE. Medical care provided to a patient when referred by one health professional to another with more specialized qualifications or interests. There are two levels of referred care: secondary and tertary. Secondary care is usually provided by a broadly skilled specialist such as a general surgeon, general internist, or obstetrician. Tertiary care is provided on referral of a patient to a subspecialist, such as an orthopedic surgeon, neurologist, or neonatologist. See also TERTATY CARE CENTER.

RETROSPECTIVE STUDY. See CASE-CONTROL STUDY.

SECONDARY CARE. See REFERRED CARE.

SENSITIVITY. The sensitivity of a diagnostic or screening test is the proportion of people who truly have a designated disorder who are so identified by the test. The test may consist of or include clinical observations.

SEQUENTIAL SAMPLE. See CONSECUTIVE SAMPLE

SPECIFICITY. The specificity of a diagnostic or screening test is the proprotion of people who are truly free of a designated disorder who are so identified by the test. The test may consist of or include clinical observations.

SURVEY. Observational or descriptive, nonexperimental study in which individuals are systematically examined for the absence or presence (or degreee of presence) of characteristics of interest.

TERTIARY CARE. See REFERRED CARE

TERTIARY CARE CENTER. A tertiary care center is a medical facility that receives referrals from both primary and secondary care levels and usually offers tests, treaatments, and procedures that are not available elsewhere. Most tertiary care centes offer a mixture of primary, secondary and tertiary care services so that it is the specific level of service rendered rather than the facility that determines the designation of care in a given study. See also REFERRED CARE.