Help to understand e i </msub> = y i </msub> &#x2212;<!--

shmilybaby4i

shmilybaby4i

Answered question

2022-06-09

Help to understand
e i = y i a x i b
e i = ( y i y ¯ ) a ( x i x ¯ ) ( b y ¯ + a x ¯ )

Answer & Explanation

lodosr

lodosr

Beginner2022-06-10Added 24 answers

Some of the notation is not fully standard, and in particular, you seem not to be distinguishing between the (unobservable) errors e i and the (observable) residuals e ^ i .
The usual notation runs like this:
- x ¯ = the average of x i , i = 1 , , n
- y ¯ = the average of y i , i = 1 , , n
- y i = a x i + b + e i , where e i is the ith error. The values of a and b are unobservable because you see only the sample ( y i , x i ), i = 1 , , n and not the whole population, and e i is unobservable because a and b are unobservable.
- a ^ and b ^ are the least-squares estimates of a and b. The least-squares estimates are observable because you can compute them base on the sample ( y i , x i ), i = 1 , , n. They satisfy y ¯ = a x ¯ + b, i.e. the least-squares line passes through the point that is the average of ( y i , x i ), i = 1 , , n.
- y ^ i = a ^ x i + b ^ = the ith "fitted value".
- e ^ i = y i y ^ i = the ith residual, not to be confused with the ith error. The residuals e ^ i are observable whereas the errors e i are not. The residuals necsessarily satisfy the two linear constraints e ^ 1 + + e ^ n = 0 and x 1 e ^ 1 + + x n e ^ n = 0, whereas the errors, on the other hand, are often taken to be independent.
- The "regression function" is x y = a x + b, whereas the fitted value y ^ i is the value of the regression function when the input is x i .

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