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I'm constructing money-neutral spread by this formula:

Spread = log(P1) - log(P2), where P1 and P2 is prices of two instruments

But sometimes spread can get into negative zone, when log(P1) < log(P2). How to avoid this and make spread always positive?

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Adding additional information may help us understand your needs. I mean: why not a simple value? –  pincopallino Apr 10 at 6:57
    
spread sign is not important. P&L will be determined by change in equity line. –  Wisent Genus Apr 10 at 7:17
    
My backtesting program accepts only data with positive quotes. I want to backtest this spread like standalone instrument. –  Eldar Agalarov Apr 10 at 8:05
    
Multiply it by $-1$. –  user2763361 Apr 10 at 8:48
    
but if spread sometimes is positive, sometimes is negative? –  Eldar Agalarov Apr 10 at 9:17
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