Statistical study and data interpretation

First find the variation unexplained by model A: 1 − R²(A). Then find its decrease: R²(B) − R²(A). The relative decrease is (R²(B) − R²(A)) / (1 − R²(A)) · 100%. Do not confuse percent with percentage points.

Do not simply add the two means and divide by 2 because the groups have different sizes. Find each group total by multiplying its mean by its number of students. Add the two totals and divide by the total number of students: (n₁m₁ + n₂m₂) / (n₁ + n₂).

First find the variation unexplained by model A: 1 − R²(A). Then find its decrease: R²(B) − R²(A). The relative decrease is (R²(B) − R²(A)) / (1 − R²(A)) · 100%. Do not confuse percent with percentage points.

Adding the constant b does not change variance. Multiplying every value by k multiplies the standard deviation by |k| and the variance by k². Therefore Var(Y) = (k · s)².

Adding the constant b does not change variance. Multiplying every value by k multiplies the standard deviation by |k| and the variance by k². Therefore Var(Y) = (k · s)².