In calculating ROI, we need to get two numbers, one for $$investment$$ and the other one for $$return$.$ In this analysis, the $$investment$$ is straightforward to measure; however, it is quite challenging to evaluate the $$return$$ that we should attribute to this particular investment. This is true especially considering that the marketing department did not stop other spending in promoting the product. Can you elaborate on the possible $($please list at least two$)$ challenges in calculating the $$return$$ on this ad spending?