What is Percentile?
A percentile is a statistical measure that indicates the value below which a certain percentage of data falls in an ordered dataset.
If a value is in the Pth percentile, it means P% of the data is less than or equal to that value.
Examples:
- Exam Scores: If you are in the 90th percentile, you scored higher than or equal to 90% of test-takers.
- Child Growth Charts: If a child is in the 75th percentile for height, 75% of children of the same age have a height less than or equal to that child’s height.
How to Calculate Percentile (Step-by-Step Explanation)
If you're looking to understand how to find the percentile manually, here’s the complete method. Here is the complete method for calculating a percentile manually
Step 1: Arrange the data in Ascending Order
Sort the dataset:
$$ x_0, x_1, x_2, x_3, \dots, x_{n-1} $$
Where:
n= total number of values
Step 2: Calculate the index position (L)
$$ L = \frac{P}{100} \times (n - 1) $$
Where:
P= desired percentile
N= total data points
Step 3: Interpret the Index (𝑟)
Case 1: If 𝑟 is an integer (r)
Percentile=X𝑟
The percentile equals the value at position r.
Case 2: If 𝑟 is not an integer
Interpolate between the surrounding values.
Let:
- i = ⌊𝑟⌋ (integer part)
- rf = 𝑟-𝑟i (fractional part)
Then calculate the percentile value using linear interpolation:
$$ p = x_i + 𝑟_f (x_{i+1} - x_i) $$
Percentile Example:
Find the 3rd percentile (P =3) of the given dataset:
10, 10, 34, 23, 54, 9
Solution:
Step 1: Sort Data in Ascending Order
9, 10, 10, 23, 34, 54
Total values:
n = 6
Desired percentile:
P = 3
Step 2: Find the Index Position (L):
Formula:
$$ L = \frac{P}{100} \times (n - 1) $$
Substitute the values:
$$ L = \frac{3}{100} \times (6 - 1) $$ $$ L = 0.03 \times 5 $$ $$ L = 0.15 $$
Step 3: Interpolate the Value
Since L=0.15, the percentile lies between position 0 and position 1:
Formula:
$$ \text{Value} = x_i + (L - \lfloor L \rfloor) \times (x_{i+1} - x_i) $$
Where:
Substitute the values:
$$ i = \lfloor 0.15 \rfloor = 0 $$ $$ p = 9 + (0.15 - 0) \times (10 - 9) $$ $$ p = 9 + 0.15 \times 1 $$ $$ p = 9.15 $$
Final Answer: The 3rd Percentile is 9.15
Common Percentiles Used in Statistics
| Percentile | Name | Meaning |
| 25th Percentile | Q1 (First Quartile) | 25% of values fall below |
| 50th Percentile | Median | Middle value |
| 75th Percentile | 3rd (Third Quartile) | 75% of values fall below |
| 90th Percentile | — | 90% of values fall below this value |
| 95th Percentile | — | 95% of values fall below this value |
| 99th Percentile | — | 99% of values fall below this value |