Can I have a user build an equation piece by piece?

Basically I have been trying to create an cumulative error code but certain values contribute more to the total error than others. My idea is that they have n values input and they would input whether each value was a pressure, a temperature, or a flow rate. and then based off those inputs it could build the error equation. For example if they said pressure it could add a sigmaX variable to the equation or if they said flow rate it could add a 2*sigma Y variable. Basically changing the equation dependant on what measurements they wanted entered into the code. Is something like this possible? I looked on the forums but couldn't quite find any situation like the problem I am having. Any help is appreciated and thanks again in advance.