The simple, common approach to estimating volatility is historical standard deviation. Here is a thread about the decision to include/exclude the mean return: https://trtl.bz/2kLRK7z
David's XLS is here: https://trtl.bz/2kOmHb6
Learning objectives: Interpret and apply the mean, standard deviation, and variance of a random variable. Calculate the mean, standard deviation, and variance of a discrete random variable. Interpret and calculate the expected value of a discrete random variable.