Normal probability paper: y-axis scaled by inverse normal CDF for plotting distribution data.

Free normal probability paper. Y-axis scaled by inverse normal CDF, turns normally-distributed data into a straight line.

Normal Probability Paper | Free Printable Statistics PDF

Normal Probability Paper

Sort your N data points in ascending order. For each point i (counting from 1), the percentile is approximately (i - 0.5) / N × 100. Plot each data value on the horizontal axis against its percentile on the vertical axis.

Rank your data first

If your data isn't normal, the type of curve tells you why. S-shaped curves indicate light tails; reverse-S indicates heavy tails; consistent upward or downward curve indicates skewness. Outliers show as isolated points pulled away from the main line.

Look for the shape of departure, not just deviation

For a quick read, draw a straight line through the middle 80 % of points by eye and ignore extreme tails. For a more careful fit, use least-squares regression on the central percentiles where the points are most reliable.

Fit by eye, then by least squares

Probability paper is a fast visual indicator, not a statistical test. If a decision rests on whether data is normally distributed (regulatory submission, scientific publication), confirm with Shapiro-Wilk or Anderson-Darling in software.