I have daily log return from 01.01.2011 to 10.28.2011 and I'd like to compare the total return of that 10 months period (which is of -7.093%) to annual log returns of previous years. I know it's really a simple question but I want to be sure not to make any mistake. Thanks in advance.
Log returns are additive. Just add the daily returns together. If you only have one average daily return you annualize simply by multiplying with an annualization factor. Often 252 is used but it depends on your specific use case. In your case you may want to multiply by (days per year / days in 10 months period) Make sure you do not apply the same to volatility. Volatility scales with the square root of time. If you want to know why then Google for properties of Brownian motion.