Calculates the main statistics of a linear regression: the slope and intercept of the line, the coefficient of determination, also known as r-square, the standard error of the slope, and the p-value for the regression. This function differs from `linear-regression-brief' in that it takes summary variables: `x' and `y' are the sums of the independent variable and dependent variables, respectively; `x2' and `y2' are the sums of the squares of the independent variable and dependent variables, respectively; and `xy' is the sum of the products of the independent and dependent variables. You should first look at your data with a scatter plot to see if a linear model is plausible. See the manual for a fuller explanation of linear regression statistics.