Mar 10

# Percentiles

Percentiles are measure of relative standing of an observation within a data. Percentiles divides a set of observations into 100 equal parts, and percentile scores are frequently used to report results from national standardized tests such as NAT, GAT etc.

The pth percentile is the value Y(p) in order statistic such that p percent of the values are less than the value Y(p) and (100-p) percent of the values are greater Y(p) . The 5th percentile is denoted by P5 , the 10th by P10 and 95th by P95 .

## Percentiles for the ungrouped data

To calculate percentiles for the ungrouped data, adopt the following procedure

1. Order the observation
2. For the mth percentile, determine the product $\frac{m.n}{100}$. If $\frac{m.n}{100}$ is not an integer, round it up and find the corresponding ordered value and if $\frac{m.n}{100}$ is an integer, say k, then calculate the mean of the Kth and (k+1)th ordered observations.

Example: For the following height data collected from students find the 10th and 95th percentiles. 91, 89, 88, 87, 89, 91, 87, 92, 90, 98, 95, 97, 96, 100, 101, 96, 98, 99, 98, 100, 102, 99, 101, 105, 103, 107, 105, 106, 107, 112.

Solution: The ordered observations of the data are 87, 87, 88, 89, 89, 90, 91, 91, 92, 95, 96, 96, 97, 98, 98, 98, 99, 99, 100, 100, 101, 101, 102, 103, 105, 105, 106, 107, 107, 112.

$P_{10}= \frac{10 \times 30}{100}=3$

So the 10th percentile i.e  P10 is 3rd observation in sorted data is 88, means that 10 percent of the observations in data set are less than 88.

$P_{95}=\frac{95 \times 30}{100}=28.5$

29th observation is our 95th percentile i.e. P95=107.

## Percentiles for the Grouped data

The mth percentile for grouped data is

$P_m=l+\frac{h}{f}\left(\frac{m.n}{100}-c\right)$

Like median, $\frac{m.n}{100}$ is used to locate the mth percentile group.

l     is the lower class boundary of the class containing the mth percentile
h   is the width of the class containing Pm
f    is the frequency of the class containing
n   is the total number of frequencies Pm
c    is the cumulative frequency of the class immediately preceding to the class containing Pm

Note that 50th percentile is the median by definition as half of the values in the data are smaller than the median and half of the values are larger than the median. Similarly 25th and 75th percentiles are the lower (Q1) and upper quartiles (Q3) respectively. The quartiles, deciles and percentiles are also called quantiles or fractiles.

Example: For the following grouped data compute P10 , P25 , P50 , and P95 given below.Solution:

1. Locate the 10th percentile (lower Deciles i.e. D1)by $\frac{10 \times n}{100}=\frac{10 \times 3o}{100}=3$ observation.
so, P10 group is 85.5–90.5 containing the 3rd observation
\begin{align*}
P_{10}&=l+\frac{h}{f}\left(\frac{10 n}{100}-c\right)\\
&=85.5+\frac{5}{6}(3-0)\\
&=85.5+2.5=88
\end{align*}
2. Locate the 25th percentile (lower Quartiles i.e. Q1)  by $\frac{10 \times n}{100}=\frac{25 \times 3o}{100}=7.5$ observation.
so, P25 group is 90.5–95.5 containing the 7.5th observation
\begin{align*}
P_{25}&=l+\frac{h}{f}\left(\frac{25 n}{100}-c\right)\\
&=90.5+\frac{5}{4}(7.5-6)\\
&=90.5+1.875=92.375
\end{align*}
3. Locate the 50th percentile (Median i.e. 2nd quartiles, 5th deciles) by $\frac{50 \times n}{100}=\frac{50 \times 3o}{100}=15$ observation.
so, P50 group is 95.5–100.5 containing the 15th observation
\begin{align*}
P_{50}&=l+\frac{h}{f}\left(\frac{50 n}{100}-c\right)\\
&=95.5+\frac{5}{10}(15-10)\\
&=95.5+2.5=98
\end{align*}
4. Locate the 95th percentile by $\frac{95 \times n}{100}=\frac{95 \times 3o}{100}=28.5$th observation.
so, P95 group is 105.5–110.5 containing the 3rd observation
\begin{align*}
P_{95}&=l+\frac{h}{f}\left(\frac{95 n}{100}-c\right)\\
&=105.5+\frac{5}{3}(28.5-26)\\
&=105.5+4.1667=109.6667
\end{align*}

The percentiles and quartiles may be read directly from the graphs of cumulative frequency function.

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Aug 23

# Deciles (Measures of Positions)

The deciles are the values (nine in numbers) of the variable that divide an ordered (sorted, arranged) data set into ten equal parts so that each part represents 1/10 of the sample or population. Deciles are denoted by D1D2, D3,…D10, where First decile (D1) is the value of order statistics that exceeds 1/10 of the observations and less than the remaining 9/10 and the D9 (ninth decile) is the value in order statistic that exceeds 9/10 of the observations and is less than 1/10 remaining observations. Note that the fifth deciles is equal to median. The deciles determine the values for 10%, 20%… and 90% of the data.

## Calculating Deciles for ungrouped Data

To calculate deciles for the ungrouped data, first order the all observation according to the magnitudes of the values, then use the following formula for mth decile.

$D_m= m \times \left( \frac{(n+1)}{10} \right) \mbox{th value; } \qquad \mbox{where} m=1,2,\cdots,9$

Example: Calculate 2nd and 8th deciles of following ordered data 13, 13,13, 20, 26, 27, 31, 34, 34, 34, 35, 35, 36, 37, 38, 41, 41, 41, 45, 47, 47, 47, 50, 51, 53, 54, 56, 62, 67, 82.
Solution:

\begin{eqnarray*}
D_m &=&m \times \{\frac{(n+1)}{10} \} \mbox{th value}\\
&=& 2 \times \frac{30+1}{10}=6.2\\
\end{eqnarray*}

We have to locate the sixth value in the ordered array and then have to more 0.2 of the distance between the sixth and seventh values. i.e. the value of 2nd decile can be calculated as
$6 \mbox{th observation} + \{7 \mbox{th observation} - 6 \mbox{th observation} \}\times 0.2$
as 6th observation is 27 and 7th observation is 31.
The second decile would be $27+\{31-27\} \times 0.2 = 27.8$

Similarly D can be calculated. D8 = 52.6.

## Calculating Deciles for grouped Data

The mth decile for grouped data (in ascending order) can be calculated from the following formula.

$D_m=l+\frac{h}{f}\left(\frac{m.n}{10}-c\right)$

where

l = is the lower class boundary of the class containing mth deciles
h = is the width of the class containing mth deciles
f = is the frequency of the class containing mth deciles
n = is the total number of frequencies
c = is the cumulative frequency of the class preceding to the class containing mth deciles

Example: Calculate the first and third deciles of the following grouped data

Solution: Deciles class for D1 can be calculated from $\left(\frac{m.n}{10}-c\right) = \frac{1 \times 30}{10} = 3$rd observation. As 3rd observation lie in first class (first group) so

\begin{eqnarray*}
D_m&=&l+\frac{h}{f}\left(\frac{m.n}{10}-c\right)\\
D_1&=&85.5+\frac{5}{6}\left(\frac{1\times30}{10}-0\right)\\
&=&88\\
\end{eqnarray*}

Deciles class for D7 is 100.5—105.5 as $\frac{7 \times 30}{10}=21$th observation which is in fourth class (group).
\begin{eqnarray*}
D_m&=&l+\frac{h}{f}\left(\frac{m.n}{10}-c\right)\\
D_7&=&100.5+\frac{5}{6}\left(\frac{7\times30}{10}-20\right)\\
&=&101.333\\
\end{eqnarray*}

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