The article "Some Field Experience in the Use of an Accelerated Method in Estimating 28-Day Strength of Concrete" (J. of Amer. Concrete Institute, 1969: 895) considered regressing y = 28day standard-cured strength (psi) against x = accelerated strength spsid. Suppose the equation of the true regression line is y = 1800 + 1.3x.
a. What is the expected value of 28-day strength when accelerated strength = 2500?
b. By how much can we expect 28-day strength to change when accelerated strength increases by 1 psi?
c. Answer part (b) for an increase of 100 psi.
d. Answer part (b) for a decrease of 100 psi.