A
P value is the probability of consistency between data and the hypothesis being tested (
38). Reporting the exact
P values (
P = 0.34 or
P = 0.02) rather than the conventional
P (
P < 0.05) is recommended for all primary analyses (
12,
37) as it conveys more information (
37). The use of the term “partially significant” or “marginally significant”, where the
P value is almost significant (e.g.
P = 0.057) is not acceptable if the significance level is defined as
P = 0.05 (
39). Some, however, argue that it is not always necessary to stick to
P = 0.05 for the interpretation of results and it is better to report the exact
P value and confidence interval for the estimator (
40).
The use of the 95% confidence interval (95% CI) can provide further information compared to
P values per se, and prefigures the direction of the effect size (negative or positive), its magnitude, and the degree of precision (
17). A confidence interval characterizes uncertainty about the true value of population parameters (
37). It is essential to provide the sample size (n) and probability values for tests of statistical significance (
13).
Statements about significance must be qualified numerically (
41). In the text, it is suggested that
P values be reported as equalities rather than as inequalities in relation to the alpha criterion (
41). In tables and figures, inequalities may be useful for groups of data (
41) where asterisks *, **, and *** are usually used to show statistical significance at 0.05, 0.01, and 0.001 probability levels, respectively (
33).
Although not consistent,
P values < 0.001 are reported as
P < 0.001; for 0.001 ≤
P values < 0.01, a three-significant digit is recommended, e.g.
P = 0.003; for 0.01 ≤
P values < 0.1, a two-significant digit is sufficient (e.g.
P = 0.05); for 0.1 ≤
P values ≤ 0.9, a one-significant digit is sufficient (e.g.
P = 0.4); and
P values > 0.9 are reported as
P > 0.9 (
42). For genome-wide association studies, the power of 10 is used for reporting
P values, e.g. 6 × 10
-9 (
42). It is generally suggested that zero be used before a decimal point when the value is below one, e.g. 0.37 (
43). According to the American Psychological Association, zero before a decimal point is used for numbers that are below one, but it can also be used for values that may exceed one (e.g. 0.23 cm). Therefore, when statistics cannot be greater than one (e.g. correlations, proportions, and
P values), do not use a zero before decimal fraction, e.g.
P = .028 not
P = 0.028 (
18); this recommendation, however, is not always adopted by everyone. The international standard is
P (large italic) although both ‘p’ and ‘P’ are allowed (
40).