A fellow student conducts a study on the serial position effect. He gives subjects a list of words and then tests their recall. He hypothesizes that words early in a list and words at the end of a list will be remembered better than words in the middle of the list. He calculates a regression line using word position as the predictor variable and recall as the response variable. Then, he calculates a confidence interval to test the null hypothesis about the slope of the regression line. To his surprise, he fails to reject the null. He does not understand how this can be. because he knows from the research literature that there is a vert strong relationship between word position and recall. What assumption was not satisfied in order for your fellow student to calculate the regression line?

Also, a figure is given and it is a U shaped graph with the middle of the U being (0,0) if that helps