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Polyfit with centering and scaling values

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Raj Raj
Raj Raj on 3 Oct 2014
Commented: Raj Raj on 3 Oct 2014
Hi All,
I want to output centering and scaling values from the command prompt.
Here is the sample data I took from the matlab help:
x = (0: 0.1: 2.5)';
y = erf(x);
When I tried polyfit command here is the result:
ppp = polyfit(x,y,3)
ppp =
9.284046212199847e-02 -6.231661786330390e-01 1.384345932284717e+00 -1.993673645226568e-02
Second trial with centering and scaling values as output requests gives
>> [ppp1,SSS,muu]= polyfit(x,y, 3)
ppp1 =
4.154044003296898e-02 -1.608834507201939e-01 2.001009389523068e-01 9.181275523715360e-01
SSS =
R: [4x4 double]
df: 22
normr: 3.578557910406126e-02
muu =
1.250000000000000e+00
7.648529270389177e-01
I just don't understand why the ppp is differing from ppp1.
Please let me know the mistake in my understanding... Thanks for your time.

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Accepted Answer

Mohammad Abouali
Mohammad Abouali on 3 Oct 2014
because ppp fits y to x
but ppp1 fits y to x^ where x^ = (x-mu1)./mu2;
check this code:
x = (0: 0.1: 2.5)';
y = erf(x);
ppp = polyfit(x,y,3)
ppp =
0.0928 -0.6232 1.3843 -0.0199
[ppp1,SSS,muu]= polyfit(x,y, 3)
ppp1 =
0.0415 -0.1609 0.2001 0.9181
plot(x,y,'r.')
hold on
plot(x,polyval(ppp,x))
plot(x,polyval(ppp1,x),'k')
plot(x,polyval(ppp1,(x-muu(1))./muu(2)),'g')
legend('Data','ppp','polyval(ppp1,x)','polyval(ppp1,(x-muu(1))./muu(2))');

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