When you say that an investment like a stock market index fund has an
expected return of 9%, you're saying that in any year there is a chance that
your return will be better than 9% and a chance that it will be worse. To get
more specific about your chances, you need to specify the expected volatility of
the investment, as well as its expected return.
The volatility of an investment is given by the statistical measure known as the standard
deviation of the return rate. You can just think of standard deviation as being synonymous with
volatility. An S&P 500 index fund has a standard deviation of about 15%; a
standard deviation of zero would mean an investment has a return rate that never
varies, like a bank account paying compound interest at a guaranteed rate.