Question on portfolio volatility

[Anonymous]

Say I have a portfolio which itself contains 10 sub-portfolios. On a daily basis I calculate the total return for each of the sub-portfolios, then use this data to calculate the overall portfolio total return. My question is this.

If I want to calculate the volatility of the overall portfolio can I use the overall portfolio historical returns or must I construct a variance-covariance matrix based on the sub-portfolio historical returns and use that ?

if the answer to the above question is that I can use the overall historical returns, does that mean
that the two methods would produce identical volatilities ?

[Author]

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