The article “Readability of Liquid Crystal Displays: A Response Surface” (Human Factors [1983]: 185–190) used an estimated regression equation to describe the relationship between y = error percentage for subjects reading a four-digit liquid crystal display and the independent variables x₁ = level of backlight, x₂ = character subtense, x3 = viewing angle, and x4 = level of ambient light. From a table given in the article, SSRegr = 19.2, SSResid = 20.0, and n = 30.

a. Does the estimated regression equation specify a useful relationship between y and the independent variables? Use the model utility test with a .05 significance level.

b. Calculate R² and se for this model. Interpret these values.

c. Do you think that the estimated regression equation would provide reasonably accurate predictions of error percentage? Explain.