On 27/05/2017 9:28 PM, GlenB wrote:
Bug: stats::line() does not produce correct Tukey line when n mod 6 is 2 or
3
Example: line(1:9,1:9) should have intercept 0 and slope 1 but it gives
intercept -1 and slope 1.2
Trying line(1:i,1:i) across a range of i makes it clear there's a cycle of
length 6, with four of every six correct.
Bug has been present across many versions.
The machine I just tried it on just now has R3.2.3:
If you look at the source (in src/library/stats/src/line.c), the
explanation is clear: the x value is chosen as the 1/6 quantile
(according to a particular definition of quantile), and the y value is
chosen as the median of the y values where x is less than or equal to
the 1/3 quantile. Those are different definitions (though I think they
would be asymptotically equivalent under pretty weak assumptions), so
it's not surprising the x value doesn't correspond perfectly to the y
value, and the line ends up "wrong".
So is it a bug? Well, that depends on Tukey's definition. I don't have
a copy of his book handy so I can't really say. Maybe the R function is
doing exactly what Tukey said it should, and that's not a bug. Or maybe
R is wrong.
Duncan Murdoch
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