N

N of 1 randomised trial
A randomised trial in an individual to determine the optimum treatment for that individual. The individual is given repeated administrations of experimental and control interventions (or of two or more experimental treatments), with thethe treatments being randomised.

Negative association
See association.

Negative study
A term often used to refer to a study with results that either do not indicate a beneficial effect of treatment or that have not reached statistical significance.  The
term can generate confusion because it can refer to either statistical significance or the direction of effect. Studies often have multiple outcomes, the criteria for
classifying studies as ‘negative’ are not always clear and, in the case of studies of risk or undesirable effects, ‘negative’ studies are ones that do not show a harmful effect.

NNH
See number needed to treat to harm.

NNT
See number needed to treat to benefit.

NNTb
See number needed to treat to benefit.

NNTh
See number needed to treat to harm.

Non-experimental study
See observational study

Non-inferiority trial
A trial designed to determine whether the effect of a new treatment is not worse than a standard treatment by more than a pre-specified amount. A one-sided version of an equivalence trial.

Non-randomised study
Any quantitative study estimating the effectiveness of an intervention (harm or benefit) that does not use randomisation to allocate units to comparison groups (including studies where ‘allocation’ occurs in the course of usual treatment decisions or peoples’ choices, i.e. studies usually called ‘observational’). To avoid ambiguity, the term should be substantiated using a description of the type of question being addressed. For example, a 'non-randomised intervention study' is typically a comparative study of an experimental intervention against some control intervention (or no intervention) that is not a randomised controlled trial. There are many possible types of non-randomised intervention study, including cohort studies, case-control studies, controlled before-and-after studies, interrupted-time-series studies and controlled trials that do not use appropriate randomisation strategies (sometimes called quasi-randomised studies).

Null hypothesis
The statistical hypothesis that one variable (e.g. which treatment a study participant was allocated to receive) has no association with another variable or set of variables (e.g. whether or not a study participant died), or that two or more population distributions do not differ from one another.  In simplest terms, the null hypothesis states that the factor of interest (e.g. treatment) has no impact on outcome (e.g. risk of death).

Number needed to harm
See number needed to treat to harm.

Number needed to treat
See number needed to treat to benefit

Number needed to treat to benefit
An estimate of how many people need to receive a treatment before one person would experience a beneficial outcome. For example, if you need to give a stroke prevention drug to 20 people before one stroke is prevented, then the number needed to treat to benefit for that stroke prevention drug is 20. The NNTb is estimated as the reciprocal of the absolute risk difference.  (Also called NNT, NNTB, number needed to treat.)

Number needed to treat to harm
A number needed to treat to benefit associated with a harmful effect. It is an estimate of how many people need to receive a treatment before one more person would experience a harmful outcome or one fewer person would experience a beneficial outcome. (Also called NNH, NNTH, number needed to harm.) See also number needed to treat to benefit.