JAMAevidence Glossary

Terms are derived from Users' Guides to the Medical Literature: A Manual for Evidence-Based Practice, 2nd Edition and The Rational Clinical Examination: Evidence-Based Clinical Diagnosis. Updated December 2009.
Download a PDF of the glossary (179 KB). (Adobe Reader is required to open or print PDF files.)





N-of-1 randomized controlled trial
An experiment designed to determine the effect of an intervention or exposure on a single study participant. In one N-of-1 design, the patient undergoes pairs of treatment periods organized so that 1 period involves the use of the experimental treatment and 1 period involves the use of an alternate treatment or placebo. The patient and clinician are blinded, if possible, and outcomes are monitored. Treatment periods are replicated until the clinician and patient are convinced that the treatments are definitely different or definitely not different.

Narrative review
A review article (eg, a typical book chapter) that is not conducted using methods to minimize bias (in contrast to a systematic review).

Natural history
As distinct from prognosis, natural history refers to the possible consequences and outcomes of a disease or condition and the frequency with which they can be expected to occur when the disease condition is untreated.

Negative effect
As clinical studies accumulate, it is more common for effects to shrink than to increase. Negative effects are smaller or less dramatic than effects from previous studies.

Negative predictive value
See Predictive value.

Negative studies
Studies in which the authors have concluded that the comparison groups do not differ statistically in the variables of interest. Research results that fail to support the researchers’ hypotheses.

Neural network
The application of nonlinear statistics to pattern-recognition problems. Neural networks can be used to develop clinical prediction rules. The technique identifies those predictors most strongly associated with the outcome of interest that belong in a clinical prediction rule and those that can be omitted from the rule without loss of predictive power.

Nominal variable
A variable that can be classified into a category (eg, male or female sex); often called categorical varaible. A categorical variable may be nominal or ordinal. Categorical variables can be defined according to attributes without any associated order (eg, medical admission, elective surgery, or emergency surgery); these are called nominal variables. A categorical variable can also be defined according to attributes that are ordered (eg, height such as high, medium, or low); these are called ordinal variables.

Nomogram
Graphic scale facilitating calculation of a probability. The most-used nomogram in the evidence-based medicine world is one developed by Fagan to move from a pretest probability, through a likelihood ratio, to a posttest probability.

Nonadherent
Patients are nonadherent if they are not exposed to the full course of a study intervention (eg, most commonly, they do not take the prescribed dose or duration of a drug or they do not participate fully in the study program).

Nonrandomized controlled trial
In an nonrandomized controlled trial, it is difficult or impossible to assign participants to different study arms (interventions) by chance. These types of trials are subject to bias because of the inability to control for factors between groups that could affect outcomes.

Null hypothesis
In the hypothesis-testing framework, this is the starting hypothesis that the statistical test is designed to consider and possibly reject, which contends that there is no relationship between the variables under study.

Null result
A nonsignificant result; no statistically significant difference between groups.

Number needed to harm
The number of patients who, if they received the experimental intervention, would lead to one additional patient being harmed over a specific period of time. It is the inverse of the absolute risk increase (ARI), expressed as a percentage (100/ARI). See also Absolute risk increase.

Number needed to screen
The number of patients who would need to be screened to prevent one adverse event.

Number needed to treat
The number of patients who need to be treated over a specific period of time to achieve one additional good outcome. When discussing NNT, it is important to specify the intervention, its duration, and the desirable outcome. It is the inverse of the absolute risk reduction (ARR), expressed as a percentage (100/ARR). See also Absolute risk reduction.