Nice work, Amanda.

Hopefully Wolfram will include more of these standard statistical tests in futures releases, to bring MMA to parity with comparable math/stats software packages.

I have written a few posts about using the Kalman filter approach in pairs trading (stat arb), for instance here and here.

I would certainly be interested to get your own take on the subject and any trading results you might care to share.

Amanda, I think you may have hit on something very important. As you point out, the determination of the variance/covariances is critical and the adaptive tuning procedure you recommend appears very successful in stabilizing the portfolio, making it suitable for a stat-arb strategy.

As you saw, I did not use MMA in my own implementation because I felt that Wolfram's approach was somewhat unsympathetic to the needs of the economic researcher (vs. say the requirements of an engineer), compared to the available alternatives. I see that I am not entirely alone in that assessment: here, for instance. So I am delighted that you have successfully implemented this in MMA, presumably using KalmanEstimator(?). Or did you build the model from scratch?

I will run a few tests on your Johansen code and attempt to build a KF model in MMA using some of the ETF pairs/triplets Ernie discusses in his book and compare the results.

Meanwhile, I wondered if you could comment on the following:

1) While the initial trading performance appear very encouraging, what kind of performance results did the backtest produce, out of sample?

2) You mention that you update the model using weekly data and then trade it intraday during the following week. So presumably you are getting real-time market data into MMA somehow: via the Finance Platform, perhaps? And do you trade the signals via that platform, or some other way (manually)?

3) One extension that i found quite useful in my own research was to fit a GARCH model to the residuals and use this to determine the trade entry/exit points. But that procedure was probably only useful because of the nonstationarity in the portfolio returns process. If you have succeeded in dealing with that key issue at a more fundamental level, a GARCH extension is probably superfluous.

Hi Amanda,

Your approach seems very promising.

On point 2: I made the assumption that you had to be getting (quasi) real-time data into MMA somehow and indeed this turns to be the case - a creative solution to the problem.

Of course, since you are only updating the model weekly you wouldn't need to use MMA at all during the week. Some trading platforms will allow you to place bids and offers for a synthetic contract according to a simple formula, where the betas are fixed (for the week). In other cases a simple api interface is provided to something like Excel. That would enable you to recalculate the entry/exit prices automatically tick-by-tick, if you wanted to, and would also eliminate the need for manual trading as the orders could be fired into the trading platform via the api.

There are the usual practical considerations that apply to any stat arb strategy. For instance, do you try to enter passively, posting orders on the bid and ask prices of the portfolio (treating it as a single synthetic security)? Another approach is to post resting orders for the individual ETF components at appropriate price levels then cross the spread on the other ETFs if you get filled on one of them. These execution strategies tend to apply more in the case of pairs trading. For more complex strategies involving multiple securities like yours they can be very tricky to implement and traders typically cross the spread on entry and exit, which is what is you are doing, I would guess.

Another question is how to treat open positions held over a w/e when models get updated. The original exit points will likely change. So you have some options there too: exit all positions by the end of the week; maintain the original exit prices (profit target and stop loss); or recalculate exit prices for existing positions once the models get updated.

Finally, one other important issue is whether to use prices or (log) returns in your cointegration model. I suspect you are using the former, as I did in my toy illustration. But the resulting portfolios are rarely dollar neutral and hence consume margin capital. On the other hand, if you use returns and create a dollar-neutral portfolio, rebalancing becomes more of an issue. In that case I suspect you would want to rebalance the portfolio at least once a day, or according to some more sophisticated rebalancing algorithm.

So I thought it might be useful to work through an example, to try to make the mechanics clear. I'll try to do this is stages so that others can jump in along the way, if they want to.

Start with some weekly data for an ETF triplet analyzed in Ernie Chan's book:

`tickers = {"EWA", "EWC", "IGE"};
period = "Week";
nperiods = 52*15;
finaldate = DatePlus[Today, {-1, "BusinessDay"}];`

After downloading the weekly close prices for the three ETFs we divide the data into 14 years of in-sample data and 1 year out of sample:

  stockdata = 
          FinancialData[#, 
             "Close", {DatePlus[finaldate, {-nperiods, period}], finaldate, 
              period}, "DateList"] & /@ tickers;
    stockprices = stockdata[[All, All, 2]];
    isprices = Take[stockprices, All, 52*14];
    osprices = Drop[stockprices, 0, 52*14];

We then apply Amanda's JohansenTest function:

JT = JohansenTest[isprices, 2, 1]

We find evidence of up to three cointegrating vectors at the 95% confidence level:

Let's take a look at the vector coefficients (laid out in rows, in Amanda's function):

We now calculate the in-sample and out-of-sample portfolio values using the first cointegrating vector:

isportprice = (JT[[2, 1]]*100).isprices;
osportprice = (JT[[2, 1]]*100).osprices;

The portfolio does indeed appear to be stationary, in-sample, and this is confirmed by the unit root test, which rejects the null hypothesis of a unit root:

ListLinePlot[isportprice]

UnitRootTest[isportprice]

0.000232746

Unfortunately (and this is typically the case) the same is not true for the out of sample period:

ListLinePlot[osportprice]

UnitRootTest[osportprice]    

0.121912

We fail to reject the null hypothesis of unit root in the portfolio process, out of sample.

I'll press pause here before we go on to the next stage, which is Kalman Filtering.

Before we delve into the Kalman Filter model, its worth pointing out that the problem with the nonstationarity of the out-of-sample estimated portfolio values is not mitigated by adding more in-sample data points and re-estimating the cointegrating vector(s):

IterateCointegrationTest[data_, n_] := 
  Module[{isdata, osdata, JT, isportprice, osportprice},
   isdata = Take[data, All, n];
   osdata = Drop[data, 0, n];
   JT = JohansenTest[isdata, 2, 1];
   isportprice = JT[[2, 1]].isdata;
   osportprice = JT[[2, 1]].osdata;
   {UnitRootTest[isportprice], UnitRootTest[osportprice]}];

We continue to add more in-sample data points, reducing the size of the out-of-sample dataset correspondingly. But none of the tests for any of the out-of-sample datasets is able to reject the null hypothesis of a unit root in the portfolio price process:

ListLinePlot[
 Transpose@
  Table[IterateCointegrationTest[stockprices, 52*14 + i], {i, 1, 50}],
  PlotLegends -> {"In-Sample", "Out-of-Sample"}]

Amanda has correctly anticipated the direction I was headed in i.e to show that regardless of how small the size of the OOS period relative to the IS period, the Johansen procedure by itself is unable to produce a cointegrating vector capable of yielding a portfolio price process that is stationary out of sample. But her iterative Kalman Filter approach is able to cure the problem.

I don't want to gloss over this finding, because it is very important. In our toy problem we know the out-of-sample prices of the constituent ETFs, and can therefore test the stationarity of the portfolio process out of sample. In a real world application, that discovery could only be made in real time, when the unknown, future ETFs prices are formed. In that scenario, all the researcher has to go on are the results of in-sample cointegration analysis, which demonstrate that the first cointegrating vector consistently yields a portfolio price process that is very likely stationary in sample (with high probability).

The researcher might understandably be persuaded, wrongly, that the same is likely to hold true in future. Only when the assumed cointegration relationship falls apart in real time will the researcher then discover that it's not true, incurring significant losses in the process, assuming the research has been translated into some kind of trading strategy.

A great many researchers have been down exactly this path, learning this important lesson the hard way. Nor do additional "safety checks" such as, for example, also requiring high levels of correlation between the constituent processes add much value. They might offer the researcher comfort that a "belt and braces" approach is more likely to succeed, but in my experience it is not the case: the problem of non-stationarity in the out of sample price process persists.

For a more detailed discussion of the problem see this post: Why Statistical Arbitrage Breaks Down

I was hitherto unaware of any methodology for tackling this problem, which is why Amanda's discovery is so important. As she demonstrates in her latest post, the iterative Kalman Filter approach is capable of producing a stationary out of sample process, based on the initial estimates of the cointegrating vector derived from the Johansen procedure.

In fact, Amanda's discovery is important in two fields of econometric research: cointegration theory and the theory of Kalman Filters in modeling inter-asset relationships where, as with the Johansen procedure, KF models have traditionally suffered from difficulties associated with nonstationarity in the out of sample period.

It's a tremendous achievement.

So, despite the fact that Amanda has leapt ahead to the finish line, I shall continue to plod along because, firstly, only by implementing the methodology can I be sure that I have properly and fully understood it and, secondly, as one discovers as one progresses in the field of quantitative research, fine details are often very important. So I am hoping that Amanda will provide additional guidance if I stray too far off piste in the forthcoming exposition.

Hi Per. I got an email today notifying me of your post to this discussion. I've been trading this algorithm during this entire time, as well as having a lot of email exchanges with Jonathan Kinlay regarding how to implement both the Johansen code and the Kalman filter in a manner similar to Chan. The algorithm was working well for me until about 2 months ago when one of the components of my triplet started rising in price in a way that appears to violate the cointegration. The result was that I waited 9 weeks for mean reversion. The portfolio finally mean-reverted, but by that time I had taken a significant hit on my profits. As a result my total trading profit over this time is much smaller than it was before this event -- around 10% or so over the past 9 months.

The past 2 months are clearly an outlier as compared to past algorithm performance. I can tell that simply by looking at the variation of the z-score over the past 10 years. The average half-life for mean-reversion of the z-score was 1.17 weeks. On a few occasions it took as long as 4 or 5 weeks to mean revert. (Remember, I don't make a profit until the portfolio value -- and thus the z-score -- mean-reverts.) Over the past 2 months or so, it took 9 weeks for my portfolio to mean revert. Is the cointegration breaking down, or was this just a one-time statistical fluke?

The problem is that when I re-calculate the Kalman filter parameters each weekend, the z-score often shifts in such a way that losses can accumulate if the mean reversion is delayed for too long. For example, on the close Friday, I may show a z-score of around 1.0. Over the weekend I re-calculate the Johansen test + Kalman filter parameters, and then when I run the algo on Monday morning, the z-score is significantly lower (in the range 0.5 - 0.8), even if the market is essentially unchanged. Thus, even if mean reversion occurs that day, I don't make as much profit as I'd hoped. This isn't a big deal if the portfolio mean reverts within 3 weeks -- I still make a profit. But if mean reversion doesn't happen for more than 3 or 4 weeks, I may end up with a loss. Waiting 9 weeks for mean reversion accumulated a 12% loss, which was more than half my profit over the past 9 months.

I'm still trading with this algorithm, but I'm waiting for a higher z-score before I "pull the trigger" on my trades, in order to increase the probability of a quicker mean-reversion. This means I may miss some trades, but that's OK. Also, I scale in my buys. (Partial buy at, say, z-score = 1.0, and another partial buy at z-score = 1.5, etc.)

If you're interested in how I implement the Kalman filter -- which is significantly different than Chan -- I wrote a detailed post on StackExchange - Quantitative Finance:

Does Chan use the wrong state transition model in his Kalman filter code?

Using a careful analysis, I argued in my post that Chan uses the wrong state transition matrix, i.e., the identity matrix, in his Kalman filter. I showed how to calculate the correct state transition matrix for a cointegrated portfolio, as well as how to initialize the Kalman filter using an adaptive tuning method. I received positive feedback from the readers of the forum, and one reader emailed Ernie Chan. This precipitated an email exchange between Ernie and I. Ernie basically said that his treatment was meant to be more general and so he didn't assume cointegration. I didn't want to argue, so I let it go. (My method actually works when there's no cointegration -- you basically get the identity matrix solution that Chan uses in that case.) Chan replied that "a stationary [cointegrated] portfolio can be more profitable", and that "your analysis may be correct". I didn't press him any further. I know what I did is correct, and none of the readers of my StackExchange post criticized my analysis. I received a fair number of up-votes.

I hope this is helpful.

Per,

Thanks for you post. I suppose that large excursions from equilibrium are a risk with mean-reversion strategies. (The underlying statistics are not strictly a normal distribution, and so "fat tails" imply that large excursions occur more frequently than one might expect.)

In reply to your points:

1. I'm using the same ETF triplet for my trading because it has a high Johansen score (>> 99%), a modest half-life for mean-reversion, and the three ETFs are very liquid (which is also very important!). I can imagine that trading multiple portfolios (or perhaps larger portfolios of more than three ETFs) would likely reduce risk, but it would also increase transaction costs. My funds are limited (< $100,000), so I haven't pursued that option.

2. As to arbitrage between an ETF and its components, I would imagine that there would be only limited arbitrage opportunities (because the ETFs track their components pretty closely) which would limit profits, as you suggest. However, I would certainly expect high cointegration. It's just the small excursions from equilibrium that would limit profitability.

3. Combining strategies is probably a good idea. I hear that that's what the large quant hedge funds do. They have multiple quants pursuing different strategies, and when one strategy is not working, others are. I actually have a trend-following algorithm that I've been using with cryptocurrencies over the past seven months, so I suppose I am "combining strategies" -- even though my total investment in cryptocurrencies is small ($5,000). Unfortunately, the cryptocurrencies had an horrendous sell-off last year. Nevertheless, my algo limited my maximum drawdown to around 20% by mostly keeping me out of the market. I'm hoping that the period of relative stability in the cryptocurrencies in the past few months is a prelude to stronger prices. I'm actually starting to make a small amount of money in the cryptos.

I agree with your comments about Chan. I'm grateful that he's illuminated the basic concepts and strategies. His book inspired me to study how to best implement the Kalman filter when trading a cointegrated portfolio, which I decided to share with others. If you have difficulty implementing the Kalman filter strategy, let me know. I can help with explanations, but I won't post my Kalman filter code on a public forum. I put too much effort into that to just give it away. I'm sure you understand.

Amanda