∑ Greek letter sigma, denoting sum of. ∑x is shorthand for $$\sum_{i=1}^{n}{x_i}$$ | x | absolute value of x α probability of a Type I error; or level of hypothesis test, usually 0.05 β probability of a Type II error χ2 value from the Chi-squared distribution, see Chi-squared table CI confidence interval CR coefficient of repeatability DF degrees of freedom F value from the F-distribution, see F-Distribution critical values table K kappa n or N sample size P probability of hypothesis r Pearson correlation coefficient rs Spearman rank correlation coefficient R2 coefficient of determination RSD relative standard deviation s standard deviation s2 variance SD standard deviation SEM standard error of the mean t value from the t-distribution, see t-distribution table z value from the Normal distribution, see Normal distribution table