# Box Plot Data Fence discrempancy

I am relatively new to Knime and am testing out its stat abilities.

So I was playing with the box plot. Nice and simple to use. Here is data output. In the documentation it says the smallest and largest value (which I refer to as inner fences) are calculated as follows: Q1 - (1.5 * IQR) or Q3 + (1.5 * IQR). So far so good.

However, when I do my own calculations my values differ. The formula is the same, values are different.

Box plot node computes the largest (upper inner fence) as 1599.
-using its documented formula for the largest
=1003 + (1.5 * (1003-540)) = 1697.5 NOT the reported 1599.

Any insights on why this should be?

hi @nbt1032b,

What youâ€™re interested in are the whisker boundaries and your formula shows the Inter-Quartil Range method. However, the whisker boundaries are actual points/observations, which should be the nearest to your calculated boundaries. That means between the Q3 and 1697,5 your point 1599 is the maximum.
For the lower whisker boundary, thatâ€™s even more obvious, as
`540 - (1,5 * (1003-540)) = -154,5`
and your overall minimum is 29 (=smallest/lower whisker boundary)

Here is a detailled definition of boxplot boundaries:

see the top of page 2.

You may also check your outliers using the node â€śnumeric outliersâ€ť as it uses the 1.5 IQR method by default. Executing this, the 1599 should remain as the maximum.

Hope that helps, greetz, Tommy

2 Likes

Thx for the explanation and article. Makes total sense.

1 Like

This topic was automatically closed 7 days after the last reply. New replies are no longer allowed.