Calculating Percentiles
Message posted by Tiffani (via 209.77.70.17) on March 9, 2001 at 7:39 PM (ET)

I am trying to calculate the 60th percentile, but do not have the original data set. The only data that I have to work with are n=798, mean=3883, std dev=1365, 25th%tile=2841, median=3684, 75th%tile=4663 and 90th%tile=5824. Is it possible to calculate or estimate an accurate 60th%tile with this information? How?

READERS RESPOND:
(In chronological order. Most recent at the bottom.)

Re: Calculating Percentiles
Message posted by Phil (via 216.175.112.150) on March 10, 2001 at 11:29 AM (ET)

You do know four points on the Cumulative Distribution Function. You could plot it and see how it looks. You could estimate the 60th percentile from that.

Re: Calculating Percentiles
Message posted by Darius (via 200.23.217.10) on March 12, 2001 at 7:19 PM (ET)

Y = 1726 + 39.16* Percentil

60 percentil = 4075.6

Re: Calculating Percentiles
Message posted by Beth (via 38.178.11.17) on July 25, 2001 at 4:40 PM (ET)

Thanks for any assistance.

Re: Calculating Percentiles
Message posted by Darius (via 200.23.217.10) on July 27, 2001 at 5:22 PM (ET)

Interpolate between Median 50 (percentil) and 75 percentil.
Y = 1726 + 39.16* Percentil

60 percentil = 4075.6
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The only number I can't get is the 1726 in Darius' formula. How is 1726 calculated.

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Sorry, let me explain, I used only X1=50, Y1=3684, X2=75, Y2=4663

Y=A*X+B
A=(Y2-Y1)/(X2-X1)
B=Y1-A*X1

I used only the closest numbers in the interpolation to 60 (50 and 75), you can use all and the equation will be (using regression)
Y= 1598.551 + 44.24*X
Y(60)= 4253
But I tink the first approach is better (because you don愒 care how the distribution was before 50 and after 75 and this is offen suggested for interpolation in statistical tables).

Re: Calculating Percentiles
Message posted by Darius (via 200.23.217.10) on July 27, 2001 at 5:57 PM (ET)

Y=912.08+115.17*X-1.846*X^2+0.013*X^3

Y(60)=3985

Re: Calculating Percentiles
Message posted by desirie gamil (via 208.142.142.21) on July 31, 2001 at 10:50 PM (ET)

What's Percenting definition ?
Message posted by Patrick (via 193.250.38.53) on November 12, 2001 at 6:46 PM (ET)

Re: Calculating Percentiles
Message posted by Tomi (via 154.32.143.5) on November 13, 2001 at 9:34 PM (ET)

Conaissez-vous la distribution de fr廦uence cumulative? Les "percentiles" sont similaire. Si on a des valeurs, un quartier des valeurs sont inferieur au "25 percentile".

Par exemple:

Data: 3, 6, 7, 9, 10, 11, 13, 16, 17, 20

Il y a 10 valeurs. Le valeur "50 percentile" est 10.5 parce que 50% des valeurs sont inferieur a 10.5. Le valeur "25 percentile" est 7 parce que 25% des valeurs sont inferieur a 7. Le valeur "75 percentile" est 16 parce que 75% des valeurs sont inferieur a 16. Naturellement avec 10 valeurs les percentiles sont approximations.

Les percentiles sont utiles pour comparaison un valeur avec les valeurs normales. Par exemple, ma fille etait 3300g apres 2 semaines. Etait-elles trop petite? Elle etait inferieur au valeur 10 percentile mais superieur au valeur 5 percentile. Non, elle est petite mais n'etait pas trop petite.