Louis, That's the whole point of walk forward analysis! The goal is to enter each new OOS period with parameter values *most relevant* to what the market is about to offer. When parameter values change, it is not a sign of failure. It is a sign of adaptation to changing market conditions.
If your parameter values stay the same, that suggests that the market is continuing as it had been. If your parameter values change, that suggests that the market has changed and that your strategy has *adapted* to the change. The frequency at which your parameter values change will largely be dependant upon the length of your IS period. If you have a 10 year IS period and a 2 week OOS period, it will be a loooooooong time before your parameter values change from one OOS to the next OOS. That is why Howard advised you in your earlier thread to experiment with different IS period lengths to determine what the ideal IS period length is for your strategy. If you are unable to find a suitable IS:OOS period lengths combination, that suggests that your strategy is poor and needs to be reworked or abandoned. Mike --- In amibroker@yahoogroups.com, "Louis Préfontaine" <[EMAIL PROTECTED]> wrote: > > Hi Thomas, > > I understand what you mean and I agree with you on what you say. My concern > is more about what to do when each IS and the following OOS uses some > parameters and then each new IS-OOS uses another parameter. Moreover, all > those parameters are (as far as I know) the best-of-many and they are not > resampled, so I see a potential problem and I have trouble identifying where > my system fails because if I use a walk-forward of 2 weeks each 2 weeks has > new parameters... > > Do you understand what I mean? Sorry if I am confusing... English is not > my first language. > > Louis > > 2008/4/18, Thomas Ludwig <[EMAIL PROTECTED]>: > > > > > The problem with that is the following: let's say my signal is a MA > > > crossover, and I optimized each MA. I apply a walk-forward of 3 > > > months, and each time the MA Crossover is different. So, in the end, > > > if the OOS is worse than IS, I don't know much more because each time > > > the walk-forward was acting on different MA Crossover. > > > > > > I disagree. Look at it this way: You perform an optimization ovet your > > IS period, and then you usually select the best parameter combination > > based on the metric you chose (like K-ratio). The basic *assumtion* is > > that this "best" parameter combination gives you a trading system that > > can generalize - i.e., it will be able to be profitable even if market > > conditions change somewhat. But you can't be sure before you apply this > > optimized system to NEW data different from the IS data - that's the > > OOS period. That's exactly what walk-forward is doing. It just adds one > > more assumption: That you normally wouldn't stick to the same > > parameters over several years. That's why it walks forward though the > > time comparing the optimized result of the respective IS period with > > the NEW data of the subsequent OOS period. Now, if you compare your IS > > results with the OOS results and you find the latter ones considerably > > worse that shows that your system is not robust. Think about the 3D > > optimization graph available in Amibroker: Such a poor system would > > probably produce a 3D surface plot with big changes (spikes, drops) > > instead of stable plateaus - i.e. it is *very* dependent on the > > parameters you chose. A small change in the parameters for the tested > > period would lead to poor results as the system has only "learned" the > > market structure in that period and will fail if that changes over > > time. > > > > > > > Do you > > > understand what I mean? And because the walk-forward only shows the > > > BEST result of ONE optimized portfolio result (RAR, RRR, CAR, etc.) I > > > just can't make it better because I can't see what is result number > > > 2, or 3. In that way, optimization seems superior, but maybe I am > > > not using walk-forward correctly. That is my main concern! > > > > > > Louis > > > > > > 2008/4/17, Louis Préfontaine <[EMAIL PROTECTED]>: > > > > Hi Thomas, > > > > > > > > Do you use Walk-forward as a random optimizer like Monte Carlo > > > > Simulation or do you use it as a parameter optimization (let's say > > > > you want to know the best numbers for a MA crossover). Or maybe > > > > both? > > > > > > > > I ask this because my feeling is that if I use it only as a > > > > parameter optimization, each parameter would be tested only one > > > > time in each IS or OOS, hence this could not be significative. I > > > > tried to add a random simulation 5 times to get the best out of 5 > > > > results, but I was wondering if this was correct or simply a waste > > > > of time. > > > > > > > > Thanks, > > > > > > > > Louis > > > > > > > > 2008/4/16, Thomas Ludwig <[EMAIL PROTECTED]>: > > > > > > Thank you very much Thomas. So in fact the walk-forward > > > > > > measures the data-mining bias in some way? I will read what > > > > > > you say I should read, and I will look at chapter 20 in > > > > > > Howard's book... > > > > > > > > > > Yes, it's explained there in detail. It's great that Amibroker > > > > > now automates this process (that wasn't the case when Howard's > > > > > book was published). > > > > > > > > > > > But still, so far I get the impression that if I backtested > > > > > > let's say cross (ma,ma...) for 2000 to 2008 and I take this > > > > > > best result and it is 100% CAR, then eben if OOS is 50% CAR I > > > > > > guess there can still be place for data-ming bias (or > > > > > > curve-fitting) because the optimization was done with the best > > > > > > result. > > > > > > > > > > If your IS period is 2000-2008 I'm afraid that there is no time > > > > > left for the OOS period ;-) Both periods must not overlap! In > > > > > the book (and you can configure Amibroker accordingly) the IS > > > > > period is chosen to be 2 years and OOS is one year. If you start > > > > > the process in 2000, the first IS period would be 2000-2001 and > > > > > the first OOS period would be 2002. The second IS period is > > > > > 2001-2002 and OOS is 2003 - and so forth. In my understanding > > > > > this approach simulates the fact that you normally don't trade a > > > > > sytem with the same parameters unchanged for many years (unless > > > > > it's a really long-term system). Rather, you would re- optimize > > > > > the system every 1-2 years to adjust it to changing market > > > > > conditions. That's what the walk-forward test is actually doing. > > > > > Every new optimization is compared with the results of an OOS > > > > > period. If the OOS results are considerably worse than the IS > > > > > results this is a strong hint that the sytem will not work in > > > > > real trading. > > > > > > > > > > > Anyway, if I understand correctly what you say, the OOS will > > > > > > ALWAYS be less than IS because the IS is optimized - that is, > > > > > > it will take the best-of-100 (or 200, etc.) result, and compare > > > > > > it with a more random result that would occure in real life. > > > > > > Am I wrong? > > > > > > > > > > The OOS results are not necessarily worse - but most often they > > > > > are. I've analyzed a couple of systems (which I don't trade) that > > > > > had beautiful IS equity curves - they all failed the walk- forward > > > > > test spectacularly. So it seems they were all overfitted and were > > > > > not able to generalize. > > > > > > > > > > Greetings, > > > > > > > > > > > > > > > Thomas > > > > > > > > > > > Thanks, > > > > > > > > > > > > Louis > > > > > > > > > > > > 2008/4/16, Thomas Ludwig <[EMAIL PROTECTED]>: > > > > > > > Louis, > > > > > > > > > > > > > > in the IS period your system is optimized, then the best > > > > > > > values from the optimization are used to perform a test over > > > > > > > the IS and OOS periods. > > > > > > > > > > > > > > If the OOS results are worse than the IS results, this means > > > > > > > that the system doesn't generalize well enough. BTW: This > > > > > > > topic is very well explained in chapter 20 of Howard's book. > > > > > > > I also suggest to look at > > > > > > > > > > > > > > http://www.amibroker.com/kb/2008/02/12/getting-started- with-a > > > > > > > > >utomat ic-walk-forward-optimization/. > > > > > > > > > > > > > > > > I must say, that walk forward testing has completely changed > > > > > > > my way of thinking. It's much easier to see now if a trading > > > > > > > system is worth a second look. > > > > > > > > > > > > > > Greetings, > > > > > > > > > > > > > > Thomas > > > > > > > > > > > > > > > Hi, > > > > > > > > > > > > > > > > I've been experimenting with walking-forward, and I have > > > > > > > > some questions regarding how it works. > > > > > > > > > > > > > > > > I ran a complete random optimization or buying/selling > > > > > > > > using the variables I set (a MCS in fact), and > > > > > > > > systematically OOS results were worst than IS. I don't > > > > > > > > understand how it works, because whatever if the sampling > > > > > > > > is IS or OOS it is always the same variables that are in > > > > > > > > place. > > > > > > > > > > > > > > > > Anyone could explain how this work? > > > > > > > > > > > > > > > > Thanks, > > > > > > > > > > > > > > > > Louis > > > > > > > > > > > > > > ------------------------------------ > > > > > > > > > > > > > > Please note that this group is for discussion between users > > > > > > > only. > > > > > > > > > > > > > > To get support from AmiBroker please send an e-mail directly > > > > > > > to SUPPORT {at} amibroker.com > > > > > > > > > > > > > > For NEW RELEASE ANNOUNCEMENTS and other news always check > > > > > > > DEVLOG: http://www.amibroker.com/devlog/ > > > > > > > > > > > > > > For other support material please check also: > > > > > > > http://www.amibroker.com/support.html > > > > > > > Yahoo! Groups Links > > > > > > > > > > ------------------------------------ > > > > > > > > > > Please note that this group is for discussion between users only. > > > > > > > > > > To get support from AmiBroker please send an e-mail directly to > > > > > SUPPORT {at} amibroker.com > > > > > > > > > > For NEW RELEASE ANNOUNCEMENTS and other news always check DEVLOG: > > > > > http://www.amibroker.com/devlog/ > > > > > > > > > > For other support material please check also: > > > > > http://www.amibroker.com/support.html > > > > > Yahoo! Groups Links > > > > > > > > ------------------------------------ > > > > Please note that this group is for discussion between users only. > > > > To get support from AmiBroker please send an e-mail directly to > > SUPPORT {at} amibroker.com > > > > For NEW RELEASE ANNOUNCEMENTS and other news always check DEVLOG: > > http://www.amibroker.com/devlog/ > > > > For other support material please check also: > > http://www.amibroker.com/support.html > > Yahoo! Groups Links > > > > > > > > >
