feat: explicitly set post-fit parameter errors to 0 for fixed parameters #346
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kratsg
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Aug 9, 2022
* Manually set the uncertainties for fixed parameters to zero after minimization, and remove setting the uncertainty/step_sizes during minimization, to avoid warnings from iminuit. For iminuit v2.12.2+ "assigning zero to errors is invalid, but in the past this was not caught." This approach harmonizes with cabinetry for fixed parameters for iminuit v2.12.2+. - c.f. scikit-hep/iminuit#762 - c.f. scikit-hep/cabinetry#346 - c.f. https://iminuit.readthedocs.io/en/stable/changelog.html#july-15-2022 > fix a bug in error heuristic when parameters have negative values and prevent > assigning negative values to errors * Remove tests/test_optim.py test_step_sizes_fixed_parameters_minuit as the uncertainties/step sizes values are no longer set during minimization and so are no longer in pyhf's control.
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Starting from
iminuitversion v2.12.2, running MIGRAD will set non-zero step sizes for all parameters (including fixed parameters) and does no longer just carry through the value of0.0thatcabinetrysets for them before minimization. See scikit-hep/iminuit#762 for details. To keep post-fit parameter uncertainties for fixed parameters at zero, this now sets the relevant values to0.0after minimization. As there is no longer any benefit to setting the value to zero before the fit, the initial step size settings are removed.