# How does Prism compute the value of X at Y mid and the value of Y at Xmid?

When you enter a new user-defined equation into Prism, you also create rules to generate initial values for each parameter. These rules can use computed values like Ymin (smallest Y value), Ymid (the average of the largest and smallest Y value), etc. Two of the choices are "Value of X at Ymid" and "Value of Y at X mid".

To compute the value of Y at Xmid, Prism follows these steps:

- Compute Xmid, which is the mean of the largest and smallest X values.
- Of all the data points whose X value is smaller than Xmid, find the one with the largest X value.
- Of all the points whose X value is larger than Xmid, find the one with the smallest X value.
- Draw a straight line between those two points.
- Calculate the value of Y on that line when X equals Xmid.

The value of X at Ymid is computed in a similar way.