[R] single-pass algorithm for quantile calculation
Prof Brian D Ripley
ripley at stats.ox.ac.uk
Tue Apr 3 20:54:16 CEST 2001
On Tue, 3 Apr 2001, Vadim Ogranovich wrote:
> Dear R users, I am looking for a reference to an algorithm for estimation of
> sample quantiles which does not require bringing the whole data into memory
> (more precisely its memory complexity should be much less than linear,
> ideally constant). I realize that such an algorithm can only be approximate
> and actually quite wrong for some samples, but that's fine with me.
There is no approximately accurate algorithm (consider a sorted dataset)
that does not look at the whole file.
Try random subsampling (in whatever database holds your file, or by
bringing in chunks and sampling while in memory).
Brian D. Ripley, ripley at stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272860 (secr)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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