detrending timeseries by removing the linear function fitted by samples?

I am trying to detrend a timeseries based on one sample of the timeseries in matlab.

I basically want my sample points to be all the points between 11-22, but I am getting an error saying that the t variable has to be equal to the what tac is (which is 71).

I've tried this and this does not work:

tac = 71x1 double
t = 11:22;
d_tac_ts_2 = detrend(tac,‘SamplePoints’,t)

I get this error:

Error using detrend>checkSamplePoints (line 284)
The number of elements in the 'SamplePoints' value must equal the size of
the first argument along the first dimension.

Error in detrend>parseNV (line 266)
            s = checkSamplePoints(varargin{j+1},x);

Error in detrend>parseInputs (line 161)
    [continuity,s] = parseNV(1,nargin,continuity,s,x,varargin{:});

Error in detrend (line 50)
[x,polyDeg,bp,s,continuity,sizeX,N,isrowx,isNDx,lbp] = parseInputs(x,
varargin{:});

Error in plotting_ref_TAC_vs_TAC_PET (line 28)
   d_tac_ts_2 = detrend(tac,'SamplePoints',t)

This works, but does not do what I want:

t = 1:71;
d_tac_ts_2 = detrend(tac,‘SamplePoints’,t)

thanks so much for any help!

Answer 1

[Star Strider]

Considering that the objective is to remove a linear trend, perhaps the easiest way would be:

t = 11:22;
p = polyfit(x(t),tac(t),1);
d_tac_ts_2 = tac - polyval(p, x);

Here, ‘x’ is the original independent variable vector that defines ‘tac’ as the dependent variable.

Plotting the result would be:

figure
plot(x, tac)
hold on
plot(x, d_tac_ts_2)
hold off
grid

To plot both the original and detrended data.

[nines]

hello!

thanks so much for responding! I am now getting an error:

Error using polyfit (line 44)
X and Y vectors must be the same size.

Error in untitled (line 30)

which I tried to fix by defining x:

p = polyfit(x(t),tac(t),1);
x = (1:71); 
t = 11:22;
p = polyfit(x(t),tac(t),1);
d_tac_ts_2 = tac - polyval(p, x);

but no luck -- any suggestions?

[Star Strider]

My pleasure!

My test code to explore that was:

x = 0:70;
tac = polyval([1 -5 1],x);

t = 11:22;
p = polyfit(x(t),tac(t),1);
d_tac_ts_2 = tac - polyval(p, x);

figure
plot(x, tac)
hold on
plot(x, d_tac_ts_2)
hold off
grid

It ran without error.

[nines]

thank you!

[Star Strider]

My pleasure!

If my Answer helped you solve your problem, please Accept it!

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