Standardavvik
Praktisk sett er ikke standardavviket spesielt treffsikkert, siden finansielle tall ikke er normalfordelt. Det betyr at i virkeligheten er sannsynligheten større enn 5 % for at avkastningen havner utenfor +/- 2 standardavvik. Eller sagt med rene ord, standardavviket tenderer til å undervurdere risikoen for ekstreme utfall.
Utregning av standardavvik:
Monthly Standard Deviation
where
X1 = return for first month
Xi = return for the ith month (each month between the first and last month)
Xn = return for the last month
n = total number of returns being used, in this case 36.
= average monthly total return during the 36-month period, also called the arithmetic mean. This number is arrived at by adding together all 36 monthly returns for the fund and dividing by 36.
Annualized Standard Deviation
where
sM = Monthly standard deviation
Ri = Return of the portfolio in month i
n = Number of periods
= Average monthly total return for the portfolio
is also called the arithmetic mean, and it is calculated by adding together all the monthly returns for the portfolio and dividing by the number of months.
Morningstar annualizes the monthly standard deviation by multiplying it by the square root of 12.