# Percentile

A **percentile** (or a centile) is a measure used in statistics indicating the value below which a given percentage of observations in a group of observations fall. For example, the 20th percentile is the value (or score) below which 20 percent of the observations may be found.

The term percentile and the related term percentile rank are often used in the reporting of scores from norm-referenced tests. For example, if a score is in the 86th percentile, it is higher than 86% of the other scores. The 25th percentile is also known as the first quartile (*Q*_{1}), the 50th percentile as the median or second quartile (*Q*_{2}), and the 75th percentile as the third quartile (*Q*_{3}). In general, percentiles and quartiles are specific types of quantiles.

## Applications

When ISPs bill "burstable" internet bandwidth, the 95th or 98th percentile usually cuts off the top 5% or 2% of bandwidth peaks in each month, and then bills at the nearest rate. In this way infrequent peaks are ignored, and the customer is charged in a fairer way. The reason this statistic is so useful in measuring data throughput is that it gives a very accurate picture of the cost of the bandwidth. The 95th percentile says that 95% of the time, the usage is below this amount. Just the same, the remaining 5% of the time, the usage is above that amount.

Physicians will often use infant and children's weight and height to assess their growth in comparison to national averages and percentiles which are found in growth charts.

The 85th percentile speed of traffic on a road is often used as a guideline in setting speed limits and assessing whether such a limits is too high or low.^{[1]}

## The normal distribution and percentiles

The methods given in the Definitions section are approximations for use in small-sample statistics. In general terms, for very large populations following a normal distribution percentiles may often be represented by reference to a normal curve plot. The normal distribution is plotted along an axis scaled to standard deviations, or sigma units. Mathematically, the normal distribution extends to negative infinity on the left and positive infinity on the right. Note, however, that only a very small proportion of individuals in a population will fall outside the −3 to +3 range. For example, with human heights very few people are above the +3 sigma height level.

Percentiles represent the area under the normal curve, increasing from left to right. Each standard deviation represents a fixed percentile. Thus, rounding to two decimal places, −3 is the 0.13th percentile, −2 the 2.28th percentile, −1 the 15.87th percentile, 0 the 50th percentile (both the mean and median of the distribution), +1 the 84.13th percentile, +2 the 97.72nd percentile, and +3 the 99.87th percentile. This is known as the 68–95–99.7 rule or the three-sigma rule.{{ safesubst:#invoke:Unsubst||$N=Dubious |date=__DATE__ |$B=
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}} Note that in theory the 0th percentile falls at negative infinity and the 100th percentile at positive infinity, although in many practical applications, such as test results, natural lower and/or upper limits are enforced.

## Definitions

There is no standard definition of percentile,^{[2]}^{[3]}^{[4]}
however all definitions yield similar results when the number of observations is very large.^{[5]} Some methods for calculating the percentiles are given below.

## The Nearest Rank method

### Definition of the Nearest Rank method

One definition of percentile, often given in texts, is that the *P*-th percentile of a list of *N* ordered values (sorted from least to greatest) is the smallest value in the list such that percent of the data is less than or equal to that value. This is obtained by first calculating the ordinal rank and then taking the value from the ordered list that corresponds to that rank. The ordinal rank *n* is calculated using this formula

Note the following:

- Using the Nearest Rank method on lists with fewer than 100 values can result in the same number being used for more than one percentile.
- A percentile calculated using the Nearest Rank method will always be a member of the original ordered list.
- The 100th percentile is defined to be the largest value in the ordered list.

### First worked example of the Nearest Rank method

Consider the ordered list {15, 20, 35, 40, 50}, which contains five data values. What are the 30th, 40th, 50th and 100th percentiles of this list using the Nearest Rank method?

So the 30th, 40th, 50th and 100th percentiles of the ordered list {15, 20, 35, 40, 50} using the Nearest Rank method are {20, 20, 35, 50}

### Second worked example of the Nearest Rank method

Consider an ordered population of 10 data values {3, 6, 7, 8, 8, 10, 13, 15, 16, 20}. What are the 25th, 50th, 75th and 100th percentiles of this list using the Nearest Rank method?

So the 25th, 50th, 75th and 100th percentiles of the ordered list {3, 6, 7, 8, 8, 10, 13, 15, 16, 20} using the Nearest Rank method are {7, 8, 15, 20}

### Third worked example of the Nearest Rank method

Consider an ordered population of 11 data values {3, 6, 7, 8, 8, 9, 10, 13, 15, 16, 20}. What are the 25th, 50th, 75th and 100th percentiles of this list using the Nearest Rank method?

So the 25th, 50th, 75th and 100th percentiles of the ordered list {3, 6, 7, 8, 8, 9, 10, 13, 15, 16, 20} using the Nearest Rank method are {7, 9, 15, 20}

## The Linear Interpolation Between Closest Ranks method

### Definition of the Linear Interpolation Between Closest Ranks method

An alternative to rounding used in many applications is to use linear interpolation between the two nearest ranks. In particular, given the *N* sorted values , we define the *percent rank* corresponding to the *n*^{th} value as:

The value *v* of the *P*-th percentile may now be calculated as follows:^{[6]}

- If then we take
- If , then we take
- If for some integer
*k*, then we take . - Otherwise, we find the integer
*k*for which , and take

Note the following:

- A percentile calculated using the Linear Interpolation Between Closest Ranks method will not always be a member of the original ordered list.
- The 50th percentile calculated using the Linear Interpolation Between Closest Ranks method is just the sample median.
- When
*N*is even, the 25th percentile calculated using the Linear Interpolation Between Closest Ranks method is the median of the first values (i.e. the median of the lower half of the data).

### First worked example of the Linear Interpolation Between Closest Ranks method

Consider the ordered list {15, 20, 35, 40, 50}, which contains five data values. What are the 5th, 30th, 40th and 95th percentiles of this list using the Linear Interpolation Between Closest Ranks method? First, we calculate the percent rank for each list value.

List value vn |
Position of that value in the ordered list n |
Number of values N |
Calculation of percent rank |
Percent rank pn |
Notes |
---|---|---|---|---|---|

15 | 1 | 5 | 10 | ||

20 | 2 | 5 | 30 | ||

35 | 3 | 5 | 50 | ||

40 | 4 | 5 | 70 | ||

50 | 5 | 5 | 90 |

Then we take those percent ranks and calculate the percentile values as follows:

So the 5th, 30th, 40th and 95th percentiles of the ordered list {15, 20, 35, 40, 50} using the Linear Interpolation Between Closest Ranks method are {15, 20, 27.5, 50}

## {{safesubst:#invoke:anchor|main}}The Weighted Percentile method

### Definition of the Weighted Percentile method

In addition to the percentile function, there is also a *weighted percentile*, where the percentage in the total weight is counted instead of the total number. There is no standard function for a weighted percentile. One method extends the above approach in a natural way.

Suppose we have positive weights associated, respectively, with our *N* sorted sample values. Let

the -th partial sum of the weights. Then the formulas above are generalized by taking

and

The 50% weighted percentile is known as the weighted median.

## Microsoft Excel method

### Definition of the Microsoft Excel method

Some software packages, including Microsoft Excel^{[4]} (up to and including version 2010 by means of PERCENTIL and PERCENTIL.INC functions, 2013 unverified) use the following method, noted as an alternative by NIST^{[7]}
to estimate . Here, is the value of the *P*-th percentile of an ascending ordered dataset containing *N* elements with values .

Firstly, the rank is calculated:

Then the rank is split into its integer component *k* and decimal component *d*, such that . Then is calculated as:

### First worked example of the Microsoft Excel method

Consider the ordered list {15, 20, 35, 40, 50}, which contains five data values. What is the 40th percentile of this list using the Microsoft Excel method?

First we calculate the rank of the 40th percentile as follows:

So *n*=2.6, which gives us *k*=2 and *d*=0.6. So the value of the 40th percentile is calculated as:

So the value of the 40th percentile of the ordered list {15, 20, 35, 40, 50} using the Microsoft Excel method is 29

### Second worked example of the Microsoft Excel method

Consider the ordered list {1,2,3,4} which contains four data values. What is the 75th percentile of this list using the Microsoft Excel method?

First we calculate the rank of the 40th percentile as follows:

So *n*=3.25, which gives us *k*=3 and *d*=0.25. So the value of the 75th percentile is calculated as:

So the value of the 75th percentile of the ordered list {1,2,3,4} using the Microsoft Excel method is 3.25

## NIST method

### Definition of the NIST method

The primary method recommended by NIST^{[7]} to estimate is given below. Here, is the value of the *P*-th percentile of an ascending ordered dataset containing *N* elements with values .

Firstly, the rank is calculated:

Then the rank is split into its integer component *k* and decimal component *d*, such that . Then is calculated as:

(Adopted by Microsoft Excel 2010 by means of PERCENTIL.EXC function)

### First worked example of the NIST method

Consider the ordered list {15, 20, 35, 40, 50}, which contains five data values. What is the 40th percentile of this list using the NIST method?

First we calculate the rank of the 40th percentile as follows:

So *n*=2.4, which gives us *k*=2 and *d*=0.4. So the value of the 40th percentile is calculated as:

So the value of the 40th percentile of the ordered list {15, 20, 35, 40, 50} using the NIST method is 26

## See also

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## References

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## External links

- Free Online Software (Calculator) computes Percentiles for any dataset according to 8 different percentile definitions.
- Percentiles: Measures of Relative Standing of an observation in data set. Percentiles for grouped and ungrouped data.