Re: Median

omid omoomi wrote:
> Hi,
> I'll be glad if you describe more details about the problem. Is it a kind of
> statistical analysis or what?
> Omid Omoomi

Sorry to respond that slow.

It's something, Ulf Mehlig described to me a couple of months
ago. The median is the value, below and above of which
exactly half of the sorted items would be. Having an odd
number of entries, it's exactly the one in the middle like

1 2 3 4 5 6 7
------|------
M = 4

Having an even number of entries, it's the average between
the two in the middle like

1 2 3 4 5 6 7 8
-------|-------
M = (4 + 5) / 2

As he said, other ranges like Quartile (the position where
25% of the entries are below and 75% are above) would also be
of interest. So the most useful thing would be an aggregate
like qantil(n), where n is value between 0.0 and 100.0, so
that quantil(50.0) is the Median, quantil(25.0) is the first
Quartile and so on.

I don't see any quick solution how to solve this problem with
an aggregate. Aggregates get all selected values in unsorted
order, and don't know ahead how many items there will be.
Even if, all this wouldn't be of any use, because you need to
look at the entire sorted list of selected items.

Maybe someone else has an idea.

>
> >From: "Kermani, Bahram" <BKermani(at)illumina(dot)com>
> >To: "'pgsql-sql(at)postgresql(dot)org'" <pgsql-sql(at)postgresql(dot)org>
> >Subject: [SQL] Median
> >Date: Fri, 30 Jun 2000 17:37:06 -0700
> >
> >Hello,
> >
> >I am trying to do a Median or Trimmed-mean operation in postgreSQL. I was
> >wondering if anybody knew how to do it. I appreciate it if you reply to my
> >email address.
> >
> >Thanks,
> >Bahram Kermani
> >bkermani(at)illumina(dot)com <mailto:bkermani(at)illumina(dot)com>
>
> ________________________________________________________________________
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>

Jan

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