Often I have seen that it becomes a challenge to explain how Standard Deviation is to be understood.
Excel Formula: =STDEV.S(Select the Cells with data)
The resultant will be 1 Standard Deviation (SD).
1 SD covers 66% of the sample data
2 SD covers 95% of the sample data
Example: 10 yr Avg returns is 13% and the Std Dev is 1.98%.
1SD = 11.38% to 15.38%
Calculation
13.40+1.98 = 15.38
13.40 - 1.98 = 11.38
2SD = 9.44% to 17.36%
Calculation
13.40+(2x1.98) = 17.36
13.40 - (2x1.98) = 9.44
Meaning: 9.44% to 17.36% is the range in which 95% of the data points are over the last 10 years.
Relevant questions:
How many working days were there?
How many data points were used to derive SD?
If there is a massive discrepancy in the above data then perhaps understanding the source of the data and the calculation methodology is important.
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