> Tukey divides the points into three groups, not the x and y values separately.
> I'll try to get hold of the book for a direct quote, might take a couple of days. Ah well, I can't get it for a week. But the fact that it's often called Tukey's three group line (try a search on *tukey three group line* and you'll get plenty of hits) is pretty much a giveaway. On Mon, May 29, 2017 at 2:19 PM, GlenB <[email protected]> wrote: > Tukey divides the points into three groups, not the x and y values > separately. > > I'll try to get hold of the book for a direct quote, might take a couple > of days. > > > > On Mon, May 29, 2017 at 8:40 AM, Duncan Murdoch <[email protected]> > wrote: > >> 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 >> >> > [[alternative HTML version deleted]] ______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
