Most of people choose beta as the most difficult to

Question

Question 1
Most of people choose beta as the most difficult to estimate. Some argue that beta is easy to calculate, as it is simply a formula.

The Formula for Beta is:

beta= [Covariance(r, R_m)] / [Standard Deviation(R_m)]²

Where:
r is the return rate of the investment;
R_m is the return on the market portfolio.

How would you respond?
Write in 150 words and references.