N

Negative predictive value (NPV): The proportion of individuals with a negative test result who do not have the disease (true negative), and can be interpreted as the probability that a negative test result is correct.1, 6 Calculation: NPV = TN / (TN + FN), where TN = true negative and FN = false negative.

Negative study: A study that does not have “statistically significant” results.41 The term can generate confusion because it refers to both statistical significance and the direction of effect; studies often have multiple outcomes; the criteria for classifying studies as negative or positive 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.6

NNH: See number needed to harm.

NNT: See number needed to treat.

Nonexperimental study: See observational study.

Nonrandomized trial: A clinical trial in which subjects are assigned to treatments on other than a randomized basis.7

Nonsystematic error: Random error that is always present in measurement. Nonsystematic error can be estimated and reduced using statistical methods.42 See also random error.

Null hypothesis: The hypothesis being tested about a population, where null generally means “no difference” and thus refers to a hypothesis that no differences between groups or relationships between variables will be found.4

Number needed to harm (NNH): The average number of patients who need to be treated over a specific period of time to cause one additional undesirable outcome (or one fewer to experience a beneficial outcome) by the end of the period.1, 4, 6  It is the reciprocal of the absolute risk increase (ARI) or risk difference. Calculation: NNT = 1 / ARI. Also called number needed to treat to harm (NNTH).

Number needed to treat (NNT): The average number of patients who need to be treated over a specific period of time to promote one additional beneficial outcome (or prevent one additional undesirable outcome) by the end of the period.1, 4, 6   It is the reciprocal of the absolute risk reduction (ARR). Calculation: NNT = 1 / ARR. Also called number needed to treat to benefit (NNTB).