fix: use math.fsum in variance to avoid fp rounding when all credits are equal - #10
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Summary
PartialCreditScore.variancewas usingsum()to compute the mean, which accumulates floating-point rounding errorsum([0.8]*8)/8is not exactly0.8, so(c - m)**2is not exactly0.0, yielding a residual of ~1.2e-32 instead of 0.0math.fsum(exact summation) for both the mean and the squared-deviations sum eliminates the errorRoot cause
0.8is not exactly representable in IEEE 754;sum()accumulates that error across 8 terms.math.fsumuses an extended-precision accumulator that cancels the error exactly.Test plan
pytest tests/test_partial_credit.pypasses locally (8/8 green)