Help with "Calculate Values" Python expression (ModelBuilder)

Question

I have a model and need to compute a new value from two other values upstream, and then feed the new value to the Kernel Density tool.

Here is the formula I am trying to reproduce in the "Calculate Values" tool:

hopt formula

Excel

Here is what the formula looks like in Excel: =((2/(3*n))^(1/4))*(σ)

For example, if n = 9 and σ = 42840 it results in the correct answer of 22350

Python

Here is the formula as I currently have it in "Calculate Values Tool": ((2/(3*%Row Count%))**(1/4))*(%SD%)

It does not return the correct value. Am I missing a parenthesis somewhere?

The model, for context

• J.R: This may be related to the way python work with number, look at this question and see if it help : gis.stackexchange.com/questions/113021/…