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Why would a single trade at the beginning not equal the benchmark?

Hi!

My first algo invests all my funds into SPY on the very first day, then no further transactions. Why wouldn't my returns be identical to those of SPY, and why wouldn't the ratios be "perfect"?? For example, I get a Sharpe ratio of .34, yet my returns should be identical, I would have thought...

Many Thanks,
Pete

3 responses

The benchmark reinvests dividends.

The benchmark includes re-invested dividends. Your algorithm sits on a lot of cash that comes in from dividends. I added a "record" line to your algorithm so that you can monitor your cash. If you re-invest your dividends, you'll get a lot closer to benchmark.

As a bit of a tangent, I'll note that you'll never get exactly the same result. The benchmark has the privilege of "buying" at the open price, because the benchmark is always in the market. Your algorithm has to wait for your order to process, so it actually only goes long at the start of the 2nd bar. But that is a minuscule effect compared to the dividends.

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Exactly right Dan!
I made a modification to reinvest any loose cash and I now get an Alpha of 0 and a beta of 1 with identical returns (surprisingly ever so slightly higher than the benchmark).

I'm still curious about the sharpe. Is this actually the sharpe of the benchmark we are now seeing?