Before explaining the 90th percentile let’s see what is
percentile. A percentile 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 30th
percentile is the value below which 30 percent of the observations may be
found.

In similar way the **90th
percentile** is the value **below which
90 percent** of the observations may be found.

**How to calculated 90**^{th}
percentile?

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As we know it is the value from which 90% of the samples are
smaller so let’s find the 90^{th} percentile in simple easy steps.

**Step 1: **Sort the
values in increasing order.

**Step 2: **Eliminate
the top 10% values.

**Step 3: **The highest
value you are left with is the 90^{th} percentile.

**Example:**

Suppose we have following sample of data: 1, 5, 3, 9, 4, 7,
7, 9, 11, and 21.

Sorting the data in ascending order gives: 1, 3, 4, 5, 7, 7,
9, 11 and 21.

After removing top 10% values (10% of 10 = 1) we have 1, 3,
4, 5, 7, 7, 9 and 11.

The 90^{th} percentile is 11.

**What is the significance
of 90**^{th} percentile?

It answers the following questions:

1. What percentage of transactions have a response time less than or equal to X seconds?
2. What is the time under which 90 percent of the transactions are responding?