 Plotting position

A plotting position assigns a probability level to an order statistic for a Q-Q plot. The displayed formula is one convention, not a universal definition. R's ppoints uses $p_i=(i-a)/(n+1-2a)$, with $a=1/2$ by default for $n>10$, which gives our choice at $n=19$; for $n\le 10$ its default is $a=3/8$. NIST's normal probability plot instead describes an approximation to uniform distribution order statistic medians. State the convention when comparing plots, especially their extreme quantiles. ^[ref][ref]