As such, I agree about the point on univariate distributions.
Students also understand other similar model equations they study in science classes.
I agree that it is possible to define the concept of 'relation- ship between variables' in terms of multivariate or conditional univariate distributions. Defining the concept of 'relationships between variables' in terms of conditional expected value leads to a simpler definition than the definitions Herman proposes above because the expected- value approach replaces the complicated concept of 'distribution' with the simpler concept of 'expected value'. I thank Herman for his thought-provoking comments. These definitions suggest that the Random House lexicographers do not see much difference between the two terms. Alternatively, if there is no compelling evidence that the best line for the relationship between the two variables is not a straight line, we can fit a straight line to the data and then test the hypothesis that the slope of the line in the population is zero.