I ran into a strange issue fitting a line to a small number of data points using numpy.polyfit that I thought was worth documenting.
I ran a command of the form:
p, cov = np.polyfit(x,y,1,w,cov=True),
where x, y and w were arrays of length 3.
The command returned the correct slope and yintercept values, however the covariance matrix, cov, had strictly negative diagonal terms. This is apparantly because numpy scales the covariance matrix as described in here.
The scaling applied is a factor such that
factor = resids / (len(x)  order  2.0)
If, like me, you are making a first order polynomial fit to a dataset of 3 values, the denominator has the effect of multiplying the expected matrix by 1. If I was unlucky enough to have 4 points, it would have thrown bigger errors.
In my case, looking at the results here, I could recover the correct values just by multiplying the matrix by minus one. This is a strange weighting to apply to a small dataset  I assume it makes sense if you have many points and the developers wanted to keep numpy.polyfit consistent.
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