Re: Computing integral errors

Hi Alberto,

 it is true that the error on the integral does not work now on the function components. It requires the covariance matrix of the fit for the parameter correlations, and this one is obtained only for the global fitted function. I should probably re-implement the method with the option to pass a correlation matrix for the parameters.

However, your simple solution is to fit directly for this integral (i.e. signal counts).   You should re-define the functions of your fit as normalized functions (for example use the normalized Gaussian function) and use then N as fit parameters. In this way you get directly from the fit the values and the errors (no need to call IntegralError)

 Best Regards

 Lorenzo

On May 28, 2010, at 1:41 PM, Alberto Pulvirenti wrote:

> Dear ROOTers,
>
> I have the following problem, to which I don't see an easy solution.
> I have a histogram, which contains a peak signal plus some background.
>
> I want to extract the signal counts in the peak.
>
> Then, I do the following:
> 1) define a function which should reproduce the background
> 2) create a TF1 whose expression is Background + Gaussian
> 3) fit the whole histogram with that TF1
> 4) create two new TF1 objects containing the Background only and the Gaussian only
> 5) compute the integral of the Background TF1 in the peak
> 6) compute the integral of the histogram in the peak
> 7) obtain my counts from the subtraction of value (6) from value (7).
> 8) as a cross-check, compute the integral of the Gaussian in the peak.
>
> Now, for what concerns computing the integrals, I have no problem. Problems appear if I want to compute the error on those integrals.
> In fact, I have seen that apparently the computation of an integral error works only if I work with the TF1 object which I have used for the real fitting, but if I 'split' that global TF1 into its two components, I cannot obtain an integral error even if I initialize these components TF1 with the same parameters (with their errors) as I obtained in the fit done at point (3).
>
> Is there a way to overcome this difficulty, and compute the integral error on a component of this function?
>
> Hope I explained clearly the problem.
>
> Cheers
>
> Alberto
>
Received on Fri May 28 2010 - 16:31:33 CEST