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How can a strategy be profitable when backtested but not when actually used?


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How can a strategy be profitable when backtested but not when actually used?

  #21 (permalink)
 wcandrews 
Russellville, AR
 
Experience: Advanced
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No mechanical system--totally objective, will be profitable in the future.
They may "work" for a while and you'll think you are going to be rich.
(They would be profitable for longer in the past--1980s and 90s.)
Think that's not true? Then name one.
By the way, why are so many systems offered for sale versus actually trading them and getting rich?

Trading system analysis: Then and now

Excalibur Testing Software by Futures Truth was one of the first full-featured backtesting platforms. The software development process started in 1985 on a CROMEMCO 3 computer utilizing FORTRAN IV. The computer had a whopping 64k of RAM and used 5-1/4-inch floppy drives. Trader John Hill spent nearly $20,000 for the computer and hard drive — an enormous sum for a machine with the fraction of the computing power available today for a few hundred bucks.

Wayne Andrews, a close friend of Hill’s and a computer scientist, was instrumental in helping Hill acquire the CROMEMCO, as well as providing his own personal tick level price data for Hill to use.


I do have a little experience.

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  #22 (permalink)
 kevinkdog   is a Vendor
 
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wcandrews View Post
No mechanical system--totally objective, will be profitable in the future.
They may "work" for a while and you'll think you are going to be rich.
(They would be profitable for longer in the past--1980s and 90s.)
Think that's not true? Then name one.
By the way, why are so many systems offered for sale versus actually trading them and getting rich?

Trading system analysis: Then and now

Excalibur Testing Software by Futures Truth was one of the first full-featured backtesting platforms. The software development process started in 1985 on a CROMEMCO 3 computer utilizing FORTRAN IV. The computer had a whopping 64k of RAM and used 5-1/4-inch floppy drives. Trader John Hill spent nearly $20,000 for the computer and hard drive — an enormous sum for a machine with the fraction of the computing power available today for a few hundred bucks.

Wayne Andrews, a close friend of Hill’s and a computer scientist, was instrumental in helping Hill acquire the CROMEMCO, as well as providing his own personal tick level price data for Hill to use.


I do have a little experience.


I'd agree completely with your first sentence, if you add the word "forever" to the end of it. Mechanical systems can work for a good amount of time before they don't.

Your point is a very good one - traders should assume that at some point their strategy will stop working, and build risk management / position sizing etc. with that in mind. That way, they will avoid being wiped out by a strategy that stops working.

Some of the worst blowouts I have heard about were from traders who stubbornly held on to a method, when the market was telling them the method no longer worked.

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  #23 (permalink)
 kevinkdog   is a Vendor
 
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wcandrews View Post
By the way, why are so many systems offered for sale versus actually trading them and getting rich?

I believe it is because most are developed the wrong way. Too many rules, too much optimizing, improper data sampling and data mining, improper accounting of slippage and commission, bad assumptions about order fills,etc. Garbage In, Garbage Out as they say.


The end result is a great backtest that can sell the system to the unwary public, but will never work in the real world.

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  #24 (permalink)
 wcandrews 
Russellville, AR
 
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"The end result is a great backtest that can sell the system to the unwary public, but will never work in the real world. "

Very true. Years ago there used to be a "Club 3000." Systems used to cost $3000. Big Mike probably knows what I'm
talking about. The club critiqued the various scams, oops, I meant systems.

I am 72 and several years ago I finally figured out the "secret"--it's all about money (and also power).

Apparently there is an endless supply of "marks."

Sorry for the cynicism.

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  #25 (permalink)
 kevinkdog   is a Vendor
 
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wcandrews View Post
"The end result is a great backtest that can sell the system to the unwary public, but will never work in the real world. "

Very true. Years ago there used to be a "Club 3000." Systems used to cost $3000. Big Mike probably knows what I'm
talking about. The club critiqued the various scams, oops, I meant systems.

I am 72 and several years ago I finally figured out the "secret"--it's all about money (and also power).

Apparently there is an endless supply of "marks."

Sorry for the cynicism.

Your cynicism is VERY well founded!

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  #26 (permalink)
 
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 Big Mike 
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wcandrews View Post
Apparently there is an endless supply of "marks."

Because there is an endless supply of greed. On both the consumer end and seller side.

Mike

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  #27 (permalink)
 artemiso 
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kevinkdog View Post
There are ways to trick the backtest software. Also, if you heavily optimize, your results will never be the same going forward.

For what my view is worth, I think in the context of the poster's question, this is the most accurate answer. The issues experienced lie mostly with the software.

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  #28 (permalink)
shanemcdonald28
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Big Mike View Post
I am saying ...


My point being, I don't agree with anything you've said. Now anyone that reads my posts know that I routinely talk about all the pitfalls of backtesting, so don't misconstrue my meaning. But what you've described here is not accurate, not unless you are purposely making it inaccurate by using minute or daily data.

Mike

I am not purposely making anything inaccurate.

These are situations I have observed during live trading.
These are conclusions I have come to after comparing backtesting with live results. I observed more slippage at certain times than others.
The exchanges use sophisticated algorithms to fill all of the orders and this can push the orders around wildly in the lineup.

Do you think tick data being backtested demonstrates accurately how those orders are being filled ?
I dont think it does.

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