Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Goodness of Fit ##\chi^2/NDF##

  1. Aug 27, 2015 #1

    ChrisVer

    User Avatar
    Gold Member

    Suppose I want to find a model for a background from the data of it...
    One way is to try different fittings and compare the values of their ##\chi^2/NDF## if they're close to 1 or not.

    However what happens when two fits are really close to one? For example if I take a ##M_{\gamma \gamma}## background for a Higgs, and apply an exponential drop fit or a polynomial of deg=2 fit, I am getting values: 0.975(expon) and 0.983 (polynomial)...
    Physically I think the exponential is a better fitting function, but the statistics is telling me that the polynomial fits best...?
     
  2. jcsd
  3. Aug 27, 2015 #2

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Lies, damn lies, statistics !

    Hard to say anything sensible without something to look at. Is there a significant difference between the .975 and .983 ?
     
  4. Aug 27, 2015 #3

    ChrisVer

    User Avatar
    Gold Member

    I will post some figures and the printed results tomorrow because I don't have them in this machine.
     
  5. Aug 28, 2015 #4

    ChrisVer

    User Avatar
    Gold Member

    So here I have the plots of the background fitted with Exponential [itex] p_0 e^{p_1 x}[/itex] and Poly2 [itex]p_0 + p_1 x +p_2x^2[/itex]

    The [itex](\chi^2/NDF)_{exp}=102.4/118 \approx 0.868[/itex]
    And [itex](\chi^2/NDF)_{pol2}=104.2/117 \approx 0.8905[/itex]

    Typically I would say that the goodness of fit test tells me that both pol2 and expo are good to fit the data (compared to other tests I tried)....with pol2 being a little better , but expo being the physically motivated one.
     

    Attached Files:

    Last edited: Aug 28, 2015
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Goodness of Fit ##\chi^2/NDF##
  1. Goodness of fit tests (Replies: 2)

Loading...