Question regarding executing auto-correlation

2014 2 3
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更新时间：2014 2 6
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Hi all,
I have to stationarized my data using auto-correlation to be used in arima models. However, i have trouble getting the codes correct.
data = xlsread('20jun.xlsx');
pv = data; %144 readingfrom 20jun
t= [1:144]; %defined time points
plot(t,pv);
I have 144 data readings, resulting in 144 time point. Any advise will be appreciated. Thanks for the time.

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Mischa Kim
Mischa Kim 2014-2-3
What exactly is the problem with your code?
chean hau
chean hau 2014-2-4
编辑：chean hau 2014-2-4
Hi Mischa,
Thanks for taking the time to reply. The error i obtained is 'Vectors must be the same lengths'. Pardon my raw knowledge regarding matlab, but is it necessary to use an arima model to do acf? Thanks in advance.

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 采纳的回答

Image Analyst
Image Analyst 2014-2-4
编辑：Image Analyst 2014-2-4

0 个投票

Before plot, put there lines and tell us what it says
size(t) % Don't use a semicolon
size(pv) % Don't use a semicolon
class(t) % Don't use a semicolon
class(pv) % Don't use a semicolon
And I don't see any autocorrelation going on, just reading data from a workbook and plotting it.

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chean hau
chean hau 2014-2-4
Hi image analyst,
Thank you for taking time to read my post. Let me rephrase my poor post. I have these 144 readings(pv), and i need 144 time points to plot them. 1 time point to 1 pv reading.
size(t) is 1 144
size(pv) is 144 1
class(t,pv) are double.
I have actually tried using just one line, together with the above codes;
autocorr(pv)
which I doubt is correct, since the graph only showed 21 timepoints. Is there anyway I can rectify my codes?
Image Analyst
Image Analyst 2014-2-4
Try plot (t', pv) and try xcorr() with the 'coeffs' option. Look at the example in the help for xcorr for autocorrelation.
chean hau
chean hau 2014-2-5
编辑：chean hau 2014-2-5
Hi Image analyst,
Thanks for your input.
My codes are:.
x = xlsread('20jun.xlsx'); %144 readingfrom 20jun
t= [1:144]; %defined time points
plot(t,x);
figure;
[xc,lags] = xcorr(x,'coeff');
stem(lags,xc)
Is this acceptable? Sorry for my limited knowledge on matlab.
Image Analyst
Image Analyst 2014-2-5
I don't know. I don't have your data. Can't you tell if it works or not? Do you get the expected results?
chean hau
chean hau 2014-2-6
Hi Image analyst,
I think it works. Thank you so much for your advises. You have been a great help.

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