If your model has always been wrong

Piet, a reader who comments from time to time on this blog, hasn’t enjoyed what I’ve said about the economy recently. I’ve tried really hard, entirely ineffectively it seems, to answer his points and tease out exactly where his real problems lie.

This post by Paul Krugman talks exactly to “Piet’s views” – the deep-seated emotional views and ideologies that “must” make sense without the careful thought, analysis and model-building required. The same views that have proved almost completely ineffective at predicting anything so far.

Lots of people declared that they “just couldn’t believe” that huge budget deficits wouldn’t drive up interest rates, that “printing” lots of money wouldn’t cause runaway inflation, that slashing government spending wouldn’t have a positive effect on confidence. We know how that has turned out.

Paul doesn’t talk in this post about those who then start changing the facts that don’t agree with their views. “Inflation must be high because the Fed is printing money, but inflation isn’t high, therefore the measure of inflation must be wrong.” – even though multiple independent measures suggest the same level of inflation.

For every complex problem…

For every complex problem, there is a solution that is clear, simple and wrong.

Greece is a complex problem. Paul Krugman points to this Choose Your Own Adventure on the Greek crisis. Much like the original books, there are many, many nasty ends.

Read it, try it, explore a few paths. This is the best resource I’ve seen recently to explain exactly how bad almost all the alternatives are, and how there are no really good outcomes from this mess.

Rest of The World, watch out.

I can’t resist making a bit of noise about this

The second largest Bitcoin exchange is shutting down (possibly to reopen elsewhere).

Bitcoin experienced a rough night on Monday as TradeHill, the second-largest Bitcoin exchange, announced that it was closing its doors. In a statement, CEO Jered Kenna cited regulatory problems and the loss of $100,000 in a dispute with one of its payment processors as major factors in the decision. He has pledged to open a new site once these issues have been resolved.

So apart from the problems with Bitcoins, it turns out the security and marginal attractions of the construct are a little tarnished too.

How not to calibrate a model

Any model is a simplification of reality. If it isn’t, then it isn’t a model as rather is the reality.


Any simplified model I can imagine will also therefore not match reality exactly. The closer the model gets to the real world in more scenarios, the better it is.

Not all model parameters are created equal

Part of the approach to getting a model to match reality as closely as possible is calibration. Models will typically have a range of parameters. Some will be well-established and can be set confidently without much debate. Others will have a range of reasonable or possible values based on empirical research or theory. Yet others will be relatively arbitrary or unobservable.

We don’t have to guess these values, even for the unobservable parameters. Through the process of calibration, the outputs of our model can be matched as closely as possible to actual historical values by changing the input parameters. The more certain we are of the parameters a priori the less we vary the parameters to calibrate the model. The parameters with most uncertainty are free to move as much as possible to fit the desired outputs.

During this process, the more structure or relationships that can be specified the better. The danger is that with relatively few data points (typically) and relatively many parameters (again typically) there will be multiple parameter sets that fit the data with possibly only very limited difference in “goodness of fit” for the results. The more information we add to the calibration process (additional raw data, more narrowly constrained parameters based on other research, tighter relationships between parameters) the more likely we are to derive a useful, sensible model that not only fits out calibration data well but also will be useful for predictions of the future or different decisions.

How not to calibrate a model

Scientific American has a naive article outlining “why economic models are always wrong”. I have two major problems with the story: Continue reading “How not to calibrate a model”

Greek default?

So European politicians have more or less agreed a deal which may, more or less, push some of their problems to one side for a period. Yes, I’m not madly optimistic about this as a cure-all.  This is not the end of the Euro problems.

Part of the deal is a “50% loss for private investors”. Which is part true and part nonsense but will be an effective Greek default when enacted / agreed. (I don’t care how “voluntary” it may be, it’s a default and almost all definitions of default include restructuring of debt in any way that isn’t what was originally promised.)

Why is it only partly true? Well it’s not necessarily a “loss” for private investors. The probability of default on Greek bonds has been just about 100% for a while now. This probability of default is derived from market prices for Greek bonds and market spreads on Greek Credit Default Swaps (CDS) and an assumed Loss Given Default or Recovery Rate for investors when the bonds do default. Actual Recovery Rates vary widely, but often analysts plug in the average Recovery Rate over most of this century on unsecured debt which is around 40%.

So if market prices for Greek bonds assumed 100% default probability and a 40% recovery, then a 50% recovery doesn’t sound so bad. The potential downside is that Greece may still (need to) default on these written-down bonds at some point in the next two decades.

So the real question is what will the new probability of default be? Then we will know whether investors “took a loss” and perhaps gain the market’s view on how successful the deal really will be.

Narratives vs facts

I don’t usually write about The Final Frontier, but this article has a great parallel to what I do write about.

It’s worth reading the entire article, but the main message is that we cannot use the dream or story or fairy tale of imminent migration into space and other planets as an excuse not to deal with the very real problems we have on Earth right now. The misconceptions, Hollywood induced and otherwise, about the ease of space travel or even the extent of our current capabilities, are massive.

As with so many things, the stories that fill our society can be very different from the harsh reality.