The relation between Yi and Xi is linear through the origin The variation around the regression line is proportional to the X value, i.

The relation between Yi and Xi is linear through the origin The variation around the regression line is proportional to the X value, i.e. the spread around the regression line increases as X increases unlike an ordinary regression line where the spread is assumed to be constant in all parts of the line. In practice, plot yi vs. xi from the sample and see what type of relation exists. When can a ratio estimator be used?