## Fitting exponential

on 4 Aug 2011

### Rick Rosson (view profile)

Hi, I want to fit my data with an exponential curve. I use fit and fittype=exp. So far no problem. But now I only want to use the first 600 data points and the last 200 datapoints (every trace has 15000 datapoints) and make an exponential fit over the whole trace only using this datapoints.

Can anybody help me which methods to use?

Rick Rosson

### Rick Rosson (view profile)

on 4 Aug 2011

Are you asking for help with which methods to use for curve fitting, or are you asking for help in how to segregate the data in MATLAB into two separate sets of data?

fcarl

### fcarl (view profile)

on 4 Aug 2011

for curve fitting method only using the first and last datapoints but giving a exponential curve over the whole datapoints only using this first and last datapoints.

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### Rick Rosson (view profile)

on 4 Aug 2011

I will assume that you have the 15,000 data samples stored in the MATLAB Workspace as a 15,000 x 1 column vector called dataset.x.

`   %% Create time domain:`
```   N = size(dataset.x,1);
Fs = 125;
dt = 1/Fs;```
`   dataset.t = dt*(0:N-1)';`
`   %% Create subset:`
```   p = 600;
q = 200;
subset.x = [  dataset.x(1:p) ;  dataset.x(end-q+1:end)  ];
subset.t = [  dataset.t(1:p) ;  dataset.t(end-q+1:end)  ];```
`   %% Exponential fit:`
```   fitType = 'exp1';
myFit = fit(subset.t,subset.x,fitType);```

HTH.

Rick

fcarl

### fcarl (view profile)

on 4 Aug 2011

Thanks very much! Works well! :)

Rick Rosson

on 4 Aug 2011

Rick

the cyclist

### the cyclist (view profile)

on 4 Aug 2011

Not that I'm bitter or anything, but ... uh ... isn't this exactly what I proposed in my answer? (Except that you did the work for him?)

### the cyclist (view profile)

on 4 Aug 2011

If "x" is your explanatory variable, then it should be as simple as using x([1:600,end-199:end]), do similar for your response variable,and run fit() just as you did with "no problem" before. Or is it more complicated than that?

### fcarl (view profile)

on 4 Aug 2011

But I also want the "timepoints" between this two "epochs" to be fitted. So that an exponential function goes from timepoint 0 to timepoint 15000 only using the first values and the last values.

fcarl

### fcarl (view profile)

on 4 Aug 2011

The problem is a so called baselinedrift in my data. And I want to handle this problem by calculating a baseline. Because there are a lot of signals in the middle of the data I only want to use the beginning and the end and use this values to fit an exponential baseline over the whole data.

Rick Rosson

### Rick Rosson (view profile)

on 4 Aug 2011

Also, what is your time scale? You have 15000 points, but I assume that the time increment is not actually 1, but something much smaller. Do you know the sampling rate Fs and/or the time increment dt = 1/Fs?

fcarl

on 4 Aug 2011

8ms (125 Hz)

### fcarl (view profile)

on 4 Aug 2011

I want to give another explanation because I think it is a bit confusing: I have a data trace containing of 15.000 timepoints (every 0.008 s one frame -->125 Hz). At the beginning and the end of the trace there are no "events" so only baseline with noise. I want to fit the whole trace with an exponential function only using the first n data points and the last m datapoints. So that the points between do not play any role but the exponential function should consider the gap between because there I also need the baseline!

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