Here it is – I hope you enjoy it.

There’s one thing I wasn’t completely clear about towards the end of the talk, in the bit with the red and green bars where I discuss trans-dimensional models. The green parts are meant to represent the regions of parameter space that fit the data. The regions that *overfit* the data will be a tiny subset of the green bars, even in the complex model on the right hand side of the slides. Even if you conditioned on the model all the way on the right, you wouldn’t get overfitting unless you optimised *within* that model.

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- Former MSc student and current PhD student Oliver (Ollie) Stevenson was profiled on our department’s homepage. Ollie is continuing his work on cricket, but we have much more data (from NZ cricket!) and many more questions than we did for his MSc.
- My paper Computing Entropies with Nested Sampling was published in Entropy and featured as the cover story. Ignore the mistake in Equation 14…
- Speaking of Nested Sampling, my honours student Syarafana Abdul Rahman is rigorously testing what happens to Nested Sampling runs under imperfect MCMC moves. I might post any interesting conclusions later…
- We’re almost halfway through Semester 2 here at Auckland, and I’ve been teaching my intro Bayesian course. Inspired by some others who do it really well, I’m posting my lecture recordings online.
- Earlier this year I developed a custom-built DNest4 model for X-ray diffraction data for geophysicist Michael Rowe, and we’re currently writing that up. By the way, I’m open to this kind of collaboration in general – if you have an inference problem you want me to take a look at, feel free to get in touch and I’ll see if it’s something I can help with. I’m happy to work for co-authorship (for academic collaborators) or payment (for commercial ones).

Have a great day!

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The main result of Knuth and Skilling, which gets recycled throughout their paper, is that addition is the unique (up to regrading – a caveat I shall ignore) way of assigning a value to a thing that is composed of a combination of other things, when there is also an ordering (one thing is bigger/better/has a higher number associated with it [whatever the interpretation], than another).

When something is made up of a combination of , , and , the value associated with the combination is the sum of the values of , , and on their own:

.

Here is a function that takes some kind of object (whatever , , and are) and returns a real number, and I have assumed , , and are disjoint (don’t worry about it). The main properties you need are associativity (it doesn’t matter in what order you combine things):

and order (if thing is bigger than thing , then thing “ combined with ” is bigger than thing “ combined with “):

So far, all this is very abstract. But applications can be super trivial. For example, if I have a bag with 3 apples, a bag with 10 apples, and a bag with 14 apples, the total number of apples is 3 + 10 + 14 = 27. These numbers are consistent with the underlying ordering (which collections of apples are bigger than others) and the associative symmetry of combining collections of apples. I could even have the same three bags of apples, and apply the sum rule to different concepts about them. For example, the sum rule also applies to the masses of the apples, and (usually) the prices of the apples.

The other day I was wondering whether this had unforeseen applications. I came up with a sports example that works — until it doesn’t. What if I want to select the best possible cricket team, and I have some numbers that describe how good each player is. Maybe something like this (I’m using arbitrary made-up quality numbers here):

Player |
Quality |

Steve Smith | 95.6 |

Brendon Brewer | 1.9 |

Dale Steyn | 90.4 |

We can combine the players to build a team. The combination operator (set union) is associative — combining Smith with Brewer and then adding Steyn gives the same result as combining Brewer and Steyn and then adding Smith. That’s one of the properties needed for the sum rule. The other is order, which seems plausible here. If one player is better than another, the combinations formed from the better player are better than the combinations formed from the worse player.

So we could measure the quality of the Smith-Brewer-Steyn team by doing addition, getting the result 95.6 + 1.9 + 90.4 = 187.9. The theory would work, for example, by showing that taking me out of a team and putting in virtually anyone else who’s even played cricket at high school level will result in a better team.

But the addition theory of team selection breaks down eventually. Consider, say, a team composed of 11 Glenn McGrath clones, each with a quality of 93. The total team quality would be 1023. He was among the best of all time at bowling, but was bad at batting. This team would also score about 80 runs per innings and lose most matches as a result. Why doesn’t the theory work? The associative property still holds. It must be order that breaks down. And it does.

Let be a partial cricket team composed of 8 McGrath clones. We could combine with yet another McGrath clone. Or we could combine it with an okay batsman with a quality of 50. The order property says that since McGrath is higher quality than the okay batsman,

it should always be better to select yet another McGrath clone than it is to select an okay batsman

.

This is false. Associativity but no order implies no sum rule. When you don’t have to worry about the balance of the team, the order property is more true, and so you’d have sum rule behaviour at the beginning of the selection process, but it breaks down as you select more and more players.

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- I’ve made a little web page for sharing results of analyses I do (mostly these will be posterior samples and marginal likelihood values). I’ll aim to put things up when they’re sufficiently mature and ‘finished’, in the hope that someone might use them for actual science.
- Check out this fascinating post about an experiment on reddit, where anyone with could contribute to an image by painting one pixel at a time, but had to wait a few minutes between pixel edits. It’s amazing what emerged (via Diana Fleischman on Twitter).
- A professor at Carnegie Mellon has put a twist on multiple choice exams, by asking students to assign a probability distribution over the possible answers, and then grading them using the logarithmic score. This is sufficiently awesome that I might try it out one day. One way of improving this (and scaring students even more) would be to allow the students to assert a probability distribution that doesn’t factor into an independent distribution for each question (via Daniela Huppenkothen).

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Currently, there are two openings at lecturer level (~ assistant professor in the US system), which is appropriate for someone who’s just getting/got their PhD or has done one or two postdocs [stats is a bit different from physics, in that some people go straight from PhD to lecturer without doing postdocs]. There is also an opening for a senior lecturer or associate professor, which is more senior (everything except the very top, which is full professor).

If you are good, and think New Zealand is good, please apply! Great things about working here:

- A large department with lots of lovely people;
- Quite a few applied folks who collaborate with other disciplines;
- Auckland is a pretty awesome medium-sized (on a log scale) city which is small enough that you can access NZ outdoor activities easily if you like that, yet big enough that famous people come here.

The only downsides are the remote location and the fairly high cost of living (check out Expatistan to compare).

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Suppose I’m selling a bike on TradeMe (non-NZ readers: substitute “EBay”). Imagine I could mind-read everyone in New Zealand about their subjective value (the maximum price they would be willing to pay) for the bike. Lots of people don’t want or need the bike (they have better uses for their money), so I’d get lots of low answers. And a few would like it quite a lot, and they’ll have high values. Suppose the result is

where is the population size. A histogram might look like this:

We could make a continuous approximation to this frequency distribution, using a density function , whose integral is :

The corresponding cumulative distribution gives the number with value less than :

All good. That’s just a CDF, applied to a measure normalised to the population size , rather than 1 (as in the case of a probability distribution).

Now, a demand curve is a function which gives the quantity demanded (the number of bikes people would want to buy) as a function of price. For example, if were 20, that means 20 people would want to buy the bike if its price were $500. (I’m assuming no single person would want to buy two or more). has a negative gradient.

If I were to set the price at $100, how many people would want to buy the bike? That’s the same thing as the number of people for whom is greater than 100:

.

The same argument works for all prices, not just $100:

.

That is, the quantity demanded as a function of price is just the “complementary CDF” of people’s values. Since economists are odd, when they plot , they put on the y-axis, and call the plot a ‘demand curve’. After playing around with this idea a bit, I think I proved that a constant-elasticity demand curve corresponds to a Pareto distribution of values.

I hope this was interesting and/or useful.

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The “Rosenbrock function” is a function of two real numbers which is a bit tough for numerical optimisation methods to find the minimum of. People interested in efficient Monte Carlo methods have also used it to test their methods, usually by defining a “likelihood function” where is the Rosenbrock function. If you multiply this by a uniform distribution “prior”, then you have a nice challenging distribution to sample from.

On the Wikipedia page, there are two higher-dimensional generalisations of the Rosenbrock function, which can be used to make high dimensional sampling problems. The first is just a product of independent 2D Rosenbrock problems, so it’s a bit boring, but the second one (which the Wikipedia page calls *more involved*) is interesting. I found it gets even more interesting if you double the log likelihood (square the likelihood) to sharpen the target density.

My modified 50-dimensional Rosenbrock distribution is now one of the examples in DNest4, and I’ve also put a 10-dimensional version of it in my Haskell implementation of classic Nested Sampling (written with Jared Tobin). The problem is to sample the posterior (and calculate the marginal likelihood?) on a problem with parameters where the priors are

and the log likelihood is

.

I’m indexing from zero for consistency with my C++ and Haskell code. One interesting thing is that DNest4 seems to work on this problem when , (but needs fairly cautious numerical parameters), whereas classic NS with similar effort peters out, getting stuck in some kind of wrong mode. The “backtracking” in DNest is saving its ass. To be honest, I can’t be sure I’ve got the right answers, as I don’t know ground truth.

Running DNest on this for half an hour (= 1.3 billion likelihood evaluations! What a time to be alive) I got a log evidence estimate of and an information of nats. Some of the marginal distributions are really interesting. Here’s one with a cool long tail:

It looks even more awesome if you plot the points from the DNest4 target density, rather than just the posterior. Now it’s obvious why this problem is hard – what looks like the most important peak for most of the run ends up not being that important:

Here’s the correlation matrix of all 50 parameters:

It’s interesting that the diagonals get fatter for the later parameters. I wouldn’t have predicted that from inspection of the equation, but the symmetry that seems to be there might be an illusion because there aren’t “periodic boundary conditions”. Perhaps the ‘tail’ part of the distribution becomes more important for the later coordinates.

What do you think? Have I done something wrong, or is the run stuck somewhere misleading? Or is the problem really this freaky? I’d love to hear how other packages perform on this as well, and whether you got the same results.

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Answer: The two solutions are given by the quadratic formula

where , , and . Therefore, after some simplification, the two solutions are and .

I don’t like much how there is an ambiguity here. It seems to suggest we’d need more information if we actually wanted to know in a real problem. Sure, if we knew was positive, we’d take the positive solution, and conversely if it’s negative. But how would we ever know that?

To solve this problem, I propose an alternative methodology. Let’s assume is actually the solution. From this we can prove that *. *And since zero is greater than , we can also say *. *Yep. I propose this as a general procedure for solving quadratic equations. Using the quadratic formula, take the root with the plus sign, not the minus sign. Then use it to derive an inequality that you know for certain is true. This way, you don’t need to

rely on extra assumptions to determine which root is correct, and there is no ambiguity, unlike in the religion.

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This year I’ve been learning basic economics. It’s a cool subject. One interesting concept is “gains from trade”. The idea is that a person probably only participates in a trade if they think they’d benefit from it. If two parties are both doing this, and agree to trade, they both become better off. For example, suppose I want/need a can of Monster, and would be willing to pay up to $4.60 New Zealand Dollars for one. I go into a shop, who would be willing to sell me the Monster for as low as $3.50, but they set the price at $4.00 to make a profit.

When I buy the drink, I am 60 cents better off (because I got something I reckoned was worth $4.60 to me, but got it for $4.00), and the shop is 50 cents better off also. My 60c + their 50c = $1.10 gains from trade.

Apparently free markets maximise gains from trade. That’s cool, but it left me wondering about situations where that doesn’t seem like the right thing to do. For example, if a doctor in a private clinic could cure either 1) a dying poor person or 2) a billionaire with a broken finger, the billionaire would probably be willing to pay a lot more money, because the money doesn’t matter so much to a billionaire. The gains from trade would be high as the doctor would receive heaps of money. But treating the dying person would presumably create more wellbeing and less suffering in the universe.

So, what should we do? I decided to try to model this using tools I know. So I came up with a statistical mechanics-like model for the situation, and used DNest4 to compute the results. I assumed a situation with 100 doctors available, and 1,000 patients wanting treatment. The patients all varied in the severity of their conditions (i.e. how much wellbeing would improve if they got treatment) and their wealth. Each patient’s willingness to pay was determined by an increasing function of these two factors (wealth and severity of the health problem).

The “parameters” in DNest4 were allocations of doctors to patients; i.e., which 100 patients got treated? The “likelihood” was the gains from trade, so DNest4 found allocations that were much better, in gains-from-trade terms, than what you’d typically get from a lottery (any patient as likely to get treatment as any other). As DNest4 found high gains-from-trade solutions, I also computed the increase in subjective wellbeing. The correlation between the two is shown below (the units of the axes are arbitrary, so don’t read too much into them):

It turns out that allocations with high gains from trade are also those with high improvements in wellbeing, but the correlation isn’t perfect. That’s why emergency departments use traige nurses instead of auctions – because it’s pretty easy to tell who’s got the most severe problem. But an auction wouldn’t be as bad as you might initially guess, and would definitely outperform a lottery.

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