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I Covariance in fitting function

  1. Jun 12, 2016 #1
    Hello! I have to calculate the covariance between 2 parameters from a fit function. I found this package in Python called iminuit that did a good fit and also calculate the covariance matrix of the parameters. I tested the package on a simple function and I am not sure I understand the result. When the function I put is x^2+y^2, which has the minimum for x=y=0, I obtain ((1.0, 0.0), (0.0, 1.0)), as a covariance matrix. When I use (x-y)^2 I obtain ((250.24975024975475, 249.75024975025426), (249.75024975025426, 250.24975024975475)), as a covariance matrix. I don't understand why do I get value of covariance greater than 1 and why in the first case I get 0 on the sides and 1 on the main diagonal? It is the first time I encounter covariance so I am not sure I got it right. Thank you!
     
  2. jcsd
  3. Jun 12, 2016 #2
    The definition of covariance is
    862d47a1798266ac335e484532f30dee.gif

    If two variable are independent we would expect in the limit of large n that the covariance would be 0. If they are dependent the covariance is not limited to 1.

    Perhaps you are thinking of the correlation matrix call it Σ where the diagonal elements ∑II are always 1 and the off diagonal elements are -1 ≤ Σij ≤ 1

    Your second function does not have an unique minimum but an infinitely long trough for values x=y.
     
  4. Jun 12, 2016 #3

    mathman

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    covariance is not limited to 1. Correlation, the ratio of the covariance to the square root of the product of the variances, is limited (in magnitude) to 1.
     
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