fix IRT formula in 4PL #80
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Ah, nevermind. Realized that it's in the |
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The optimization formula in 4PL and the tutorial seems to be incorrect, it should be
sigmoid[ - disc * (ability - diff) ].Normally it wouldn't be an issue because either discs/ability/diff would just be flipped, though I believe there are some priors trying to enforce positivity? In addition I wonder how the model still kinda works when the model prediction has the correct formula.