You do have enough data to calculate a Sharpe Ratio (all you need is two values). Using more data improves its significance. You should be sure to only use comparable data, though. It appears that you may have experienced some style shift or a change in leverage used after the first two years or so. If that is the case, you should exclude those years from your calculations.
Sharpe Ratio is defined as follows:
SharpeRatio=(Average Period Return - Periodic Riskless Rate of Return)/(Standard Deviation of Period Returns)
You can use any period you wish, daily, weekly, monthly, yearly, etc. Because there is no standard value used for the Riskless Rate of Return (and most large futures accounts deposit bonds instead of cash), most people simply set it equal to zero. Note that the Sharpe Ratio with a zero riskless return is called the Information Ratio.
You should also be aware that the Sharpe Ratio has many weaknesses. For example, it misrepresents the risk associated with certain investment styles and can be manipulated by using derivatives.
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