Please see a textbook on Statistics. Please see "A Beginner's Guide to Uncertainty of Measurement" by Stephanie Bell: "how widely spread the readings are : The usual way to quantify spread is standard deviation. The standard deviation of a set of numbers tells us about how different the individual readings typically are from the average of the set... As a rule of thumb, roughly two thirds of all readings will fall between plus and minus ... one standard deviation of the average. Roughly 95% of all readings will fall within two standard deviations.."