An article on estimating 28-day strength of concrete considered regressing y = 2

Question

An article on estimating 28-day strength of concrete considered regressing y = 28-day standard-cured strength (psi) against x = accelerated strength (psi). Suppose the equation of the true regression line is y = 1830 + 1.1x.

(a) What is the expected value of 28-day strength when accelerated strength = 2480?

(b) By how much can we expect 28-day strength to change when accelerated strength increases by 1 psi?

(c) By how much can we expect 28-day strength to change when accelerated strength increases by 70 psi?

(d) By how much can we expect 28-day strength to change when accelerated strength decreases by 70 psi?

Explanation / Answer

(a)The expected value of 28-day strength when accelerated strength = 2480 is:

y=1830+1.1*2480

=4558

(b)The 28-day strength is expected to change when accelerated strength increases by 1 psi is 1.1.

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