The Question
Consider the following data concerning the demand ($y$) and price ($x$) of a consumer product
Demand $|$ 252 $\space$244$\space$ 241 $\space$ 234 $\space$ 230 $\space$ 223
Price$\space\space\space\space\space$ $|$2.00 $\space$2.20 2.40 2.60 2.80 3.00
Write the least squares prediction equation.
My attempt
I was able to find the least squares point estimates:
$b_1=$ $6\sum^6_{i=1}x_iy_i-(\sum^6_{i=1}x_i)(\sum^6_{i=1}yi)\over{6\sum^6_{i=1}x_i^2-(\sum_{i=1}^6x_i)^2}$$=$-27.71$
$b_0=\bar{y}-b_1\bar{x}=306.62$
(where $\bar{y}=$$\sum^6_{i=1}y_i\over{6}$ and $\bar{x}=$$\sum^6_{i=1}x_i\over{6}$)
Update!!
Thanks to this community, I learned that the least squares prediction equation is $\hat{y}=b_0+b_1x$ which means my equation is:
$$\hat{y}=-27.71+306.62x$$
my problem is when I model this to predict changes in demand by setting price to a value, for example 2.40, I do not seem to be able to get near the actual result.
i.e. $$\hat{y}=-27.71-306.62(2.40)\approx 708.18$$ which is nowhere near the actual demand!
Is there an error in my calculations? Or is it a problem in the math?
Thanks!
Best Answer
You've got your parameters reversed. Your estimated model is:
$$\hat{d}_i = 306.62 - 27.71p_i + \epsilon_i$$
Hence:
Your forecast of demand given a price of 2.4 would be $306.62 - 27.71*2.4 = 240.1$.
Additional comments: