Linear Algebra - Vector Spaces Quiz 6 (Least Squares)

In the least squares problem we find that

\[ A^T Ax = A^T b \]


c1=yin,c2=xiyixi2 c_1 = \frac{\sum y_i}{n} , c_2 = \frac{\sum x_i y_i }{\sum x_i^2}

where y=c1+c2xy = c_1 + c_2 x

I'm assuming this linear equation represents some kind of fitted line for the data. However, a previous step in the quiz involved setting all the xi x_i coordinates to xixˉ x_i - \bar{x} to simplify finding c1c_1 and c2c_2.

So I went ahead and tried this method on a simple set I made up of 20 pieces of data of coordinates (x,y) (x,y) :

{(1,4),(3,5),(4,3),(4,9),(6,2),(6,8),(8,6),(10,9),(11,7),(12,5),(13,11),(14,7),(15,9),(16,12),(17,8),(18,10),(18,13),(19,14),(20,13),(20,14)} \{ (1,4), (3,5) , (4,3), (4,9), (6,2), (6,8), (8,6), (10,9), (11,7), (12, 5), (13,11), (14,7), (15,9), (16,12), (17,8), (18,10), (18,13), (19,14), (20,13), (20,14)\}

A little spread out but I expect basically a positive linear relationship with y as a function of x.

Now I find with my calculator stat functions that xˉ=11.75\bar{x} = 11.75, so I set all xi x_i to xi11.75x_i - 11.75:

X={10.75,8.75,7.75,7.75,5.75,3.75,1.75,0.75,0.25,1.25,2.25,3.25,4.25,5.25,6.25,6.25,7.25,8.25,8.25} X = \{ -10.75, -8.75, -7.75, -7.75, -5.75, -3.75, -1.75, -0.75, 0.25, 1.25, 2.25, 3.25, 4.25, 5.25, 6.25, 6.25, 7.25, 8.25, 8.25 \}

My calculations from here:

c1=yin=16920=8.45c_1 = \frac{\sum y_i}{n} = \frac{169}{20} = 8.45 c2=xiyixi2=340.25725.750.46883c_2 = \frac{\sum x_i y_i}{\sum x_i^2} = \frac{340.25}{725.75} \approx 0.46883

I wind up with a line equation that looks like

y=0.46883x+8.45 y = 0.46883x + 8.45

If I try and plot this, the line sits way outside any of my data.

Did I miss some crucial step? Do the two constants change in subtracting the mean of X to begin with? Or am I misunderstanding the meaning of c1c_1 and c2c_2? As far as I can tell the quiz doesn't seem to suggest one way or another.

Note by Jeff Folster
11 months, 3 weeks ago

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