**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).