Re: How do work tercile, percentile & funcion percentile_cont() ?

With Libre Office Calc, same result than with PostgreSQL :
=CENTILE({1;2;3;4;5;6;7;8;9;10;11;12;13;14;15;16;17;18;19;20;21;22;23;24;25;26;27;28;29;30}; 0.3333333333)
gives
10.6666666657 (and NOT 10.5)

I didn't think so.

De: "PALAYRET Jacques" <jacques(dot)palayret(at)meteo(dot)fr>
À: pgsql-general(at)lists(dot)postgresql(dot)org
Envoyé: Mercredi 22 Avril 2020 10:15:26
Objet: Re: How do work tercile, percentile & funcion percentile_cont() ?

Of course, the same result with this table x(c1) for example :
SELECT c1, ntile(3) OVER(ORDER BY c1) FROM x ORDER BY c1 ;
c1 | ntile
----+-------
1 | 1
2 | 1
3 | 1
4 | 1
5 | 1
6 | 1
7 | 1
8 | 1
9 | 1
10 | 1
11 | 2
11 | 2
11 | 2
11 | 2
11 | 2
11 | 2
11 | 2
11 | 2
11 | 2
20 | 2
21 | 3
21 | 3
21 | 3
21 | 3
21 | 3
21 | 3
21 | 3
21 | 3
21 | 3
21 | 3
(30 lignes)

SELECT percentile_cont(1./3) WITHIN GROUP (ORDER BY c1) FROM x ;
percentile_cont
------------------
10.6666666666667
(1 ligne)

SELECT percentile_cont(2./3) WITHIN GROUP (ORDER BY c1) FROM x ;
percentile_cont
------------------
20.3333333333333
(1 ligne)

De: "PALAYRET Jacques" <jacques(dot)palayret(at)meteo(dot)fr>
À: pgsql-general(at)lists(dot)postgresql(dot)org
Envoyé: Mercredi 22 Avril 2020 10:00:49
Objet: How do work tercile, percentile & funcion percentile_cont() ?

Hello,

From a table x(c1) containing 30 lines with integer values (column c1) from 1 to 30 :

SELECT percentile_cont(1./3) WITHIN GROUP (ORDER BY c1) FROM x ;
percentile_cont
------------------
10.6666666666667
(1 ligne)
SELECT percentile_cont(2./3) WITHIN GROUP (ORDER BY c1) FROM x ;
percentile_cont
------------------
20.3333333333333
(1 ligne)

If ordering,
the first 10 lines (c1 from 1 to 10) have ntile(3) OVER(ORDER BY c1) = 1,
the 10 following lines (c1 from 11 to 20) have ntile(3) OVER(ORDER BY c1) = 2,
the last 10 lines (c1 from 21 to 30) have ntile(3) OVER(ORDER BY c1) = 3.
So, I though it should be :
percentile_cont(1./3) = 10.5 (the arithmetic average between 10 et 11) and not 10.6666666666667
percentile_cont(2./3) = 20.5 (the arithmetic average between 20 et 21) and not 20.3333333333333

Thank in advance for explanation

----- Météo-France -----
PALAYRET JACQUES
DCSC/MBD
jacques(dot)palayret(at)meteo(dot)fr
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