Define "variation" to a beginner How do you define the word "variation" "variability" to a complete beginner, like myself. For Example, I understand 'R' explains how strong the linear relationship between the change in X and Y. But sometimes, let's Say R=.91, people will say the linear trend model explains more than 91% of the variation in sales. Another Case is the Mean squared error (MSE) is a measure of variability in the forecast errors. Hope someone can use 'SIMPLE' words to explain it to me. Thanks a lot!