1. Validity - Validity is the degree to which the result of measurement corresponds with the true state of the phenomenon to be measured. There are two general kinds of validity:

• internal validity, the degree to which the results of an observation are correct for the patients to be studied; and

• external validity, the degree to which the results of an observation hold true in other settings. Generalizability is the other word for external validity.

2. Reliability - Reliability is the extent to which repeated measurements of a relatively stable phenomenon fall closely together. Reproducibility and precision are other words for this property.

3. Statistical significance (P-value) - A value associated with an observed test statistic that indicates the probability that a value is as extreme or more extreme than the one observed will arise by chance alone in repeated replications of a study.