 For normal data the points plotted in the QQ plot should fall approximately on a straight line, indicating high positive correlation.These plots are easy to interpret and also have the benefit that outliers are easily identified.Simple back-of-the-envelope test takes the sample maximum and minimum and computes their z-score, or more properly t-statistic
(number of sample standard deviations that a sample is above or below the sample mean), and compares it to the 68–95–99.7 rule