7.2. The heuristic happiness parameter

ARTEMIS evaluates a variety of statistical parameters relevant to the evaluation of your fit. These include the fitting metric χ², the fitting metric normalized by the degrees of freedom in the fit χ²_v, an R-factor, and the uncertainties of and correlations between the variable parameters.

In a formal sense, the most important of these parameters is χ²_v, which is the statistical parameter allowing you to distinguish between different fitting models applied to the same data. Unfortunately, χ²_v is difficult to interpret directly. In practice, the EXAFS analysis problem using FEFF is not a well-posed problem in the sense required by Gaussian statistics. This is true for a variety of reasons. In EXAFS, the signal is not ideally packed – that is, the signal is not composed of pure sine waves. In general, you cannot guarantee that errors in your fitting parameters are normally distributed. In general, we cannot actually identify and enumerate all sources of measurement error, many of which are complex and systematic, such as sample inhomogeneity, detector non-linearity, and so on. Finally, we do not know the true line shape of our signal – instead we use FEFF to approximate the lineshape. All of this is discussed further in my Advanced Topics in EXAFS Analysis talk

For all those reasons, χ²_v is rarely – if ever – close to 1 as expected for a good fit. Although χ²_v for different fitting models can be compared effectively, it is, generally speaking, not a sufficient criterion for evaluating the quality of an individual fit.

Happily, this is not an insolvable problem. After decades of analyzing EXAFS data, using FEFF, and using IFEFFIT, we know a lot about what constitutes a good fit. For instance, a good fit has a small R-factor, which is a measure of the percentage misfit between the data and theory. We know things about individual parameter types, e.g. that an E₀ is rarely more than 10 eV or less than -10 eV, that S²₀ and σ² should never be negative. We know that a robust fit does not have excessive correlation between variable parameters.

ARTEMIS offers a heuristic tool called “happiness” which is an attempt to enumerate the knowledge and experience we bring to a fit. After a fit, this heuristic parameter is evaluated and reported in the log file. It has no meaning other that to give you a general sense of how the fit came out.

Caution

The word “happiness” was chosen for this parameter because it is a silly word in a statistics context. Happiness is an ad-hoc, semantic metric. It has no basis in formal statistics. It is a functionally meaningless parameter and should NEVER be published. NEVER!

7.2.1. Evaluation of the happiness parameter

The happiness parameter begins with a value of 100, indicating complete happiness. After the fit is evaluated, a series of tests are run against the fitting results. If a test hits, it removes points from the happiness. The final evaluation is reported in the log file and used to color the Fit and plot buttons.

R-factor

Assess a penalty for an R-factor that is larger than 0.02. That is, any fit which has 2% or less misfit between data and theory is not assessed a penalty. Above that, the penalty scales linearly in R-factor up to a maximum penalty of 40.

Parameter values

After every path parameter for every path is evaluated, assess a penalty of 2 for each path parameter that fails a sanity check. The sanity checks include checking that E₀ is not too large either positively or negatively, checking that neither of S²₀ or σ² is negative, and checking that neither of σ² or ΔR is too large.

Restraints

If any restraints are placed upon the fit, assess a penalty for every restraint which evaluates to something non-zero. The penalty scales linearly with the size of the evaluated restraint.

Number of independent points

Assess a penalty if the number of guess parameters is larger than 2/3 of the total number of independent points as evaluated by the Nyquist criterion. Above the 2/3 threshold, the penalty scales linearly.

Correlations

Assess a penalty for each correlation that exceeds a cutoff value.

Penalty parameters

Assess a penalty equal to the evaluation of a penalty parameter. Penalty parameters have not yet been implemented.

7.2.2. Configuring the happiness evaluation

Note that everything above is something that Bruce made up out of whole cloth, inlcuding the parametrization. That said, every part of the happiness calculation is configurable. That means that you, the user, can tune the happiness evaluation to report on fit quality in a way that is meaningful and useful for your data and your fits.

The configuration parameters are spread over two configuration groups. The parameters in the “Happiness” group control the evaluation of the penalties. Those in the “Warning” group control the assessment of path parameter values.

For example, to tune the assessment of the penalty for an excessive number of guess parameters, you can change two configuration parameters. ♦Happiness→nidp_cutoff default to 2/3 and sets the cutoff below which no penalty is assessed. This parameter is interpreted as a fraction of total number of independent points in the fit. ♦Happiness→nidp_scale sets the maximum penalty to assess for using up all the independent points.

The path parameter penalties can also be fine tuned. A couple examples: the value above which a penalty is assessed for an excessively large E₀ is set with ♦Warnings→e0_max and the assessment of a penalty for a negative σ² can be turned off by toggling ♦Warnings→ss2_neg.

Since this happiness thing is an artificial and statistically meaningless creation, it can be changed as you like. Do you have any ideas for penalty assessments? Open an issue at the DEMETER website. Do you have a suggestion for how the existing penalties should be tuned? Post your configuration parameter values as an issue.

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