The interquartile range, or IQR for short, is an important way to understand how spread out the numbers are in a group. It also helps us find any unusual values, called outliers.

Here’s a simple breakdown:

1. What is IQR?
   We find the IQR using this formula:
   IQR = Q3 - Q1
   Here, Q1 is the first quartile (the middle value of the lower half of the data), and Q3 is the third quartile (the middle value of the upper half).

2. Finding Outliers:
   A number is seen as an outlier if it falls outside this range:
   [Q1 - 1.5 × IQR, Q3 + 1.5 × IQR]
   This means we look for data points that are much lower or much higher than most of the others.

3. Example:
   Let’s say Q1 is 10 and Q3 is 20.
   We can find the IQR like this:
   IQR = 20 - 10 = 10

   Now, to find the outlier boundaries, we do this:
   [10 - 15, 20 + 15], which gives us:
   [-5, 35]

   Any data point below -5 or above 35 is considered an outlier.

So, the IQR helps us figure out which numbers are normal and which ones are odd or unusual.