# Free statistical calculators

## Computational notes

### P-value

The significance level, or P-value, is calculated using a general z-test (Altman, 1991):

where p is the observed proportion; pexp is the Null hypothesis (or expected) proportion; and se(p) is the standard error of the expected proportion:

The P-value is the area of the normal distribution that falls outside ±z (see Values of the Normal distribution table).

### Confidence interval

MedCalc calculates the "exact" Clopper-Pearson confidence interval for the observed proportion (Clopper & Pearson, 1934; Fleis et al., 2003).

## Literature

• Altman DG (1991) Practical statistics for medical research. London: Chapman and Hall.
• Clopper C, Pearson ES (1934) The use of confidence or fiducial limits illustrated in the case of the binomial. Biometrika 26:404–413.
• Fleiss JL, Levin B, Paik MC (2003) Statistical methods for rates and proportions, 3rd ed. Hoboken: John Wiley & Sons. (p. 26)