# Bayesian

## Use domain knowledge to review prior distributions

At the Insurance Data Science conference, both Eric Novik and Paul-Christian Bürkner emphasised in their talks the value of thinking about the data generating process when building Bayesian statistical models. It is also a key step in Michael Betancourt’s Principled Bayesian Workflow. In this post, I will discuss in more detail how to set priors, and review the prior and posterior parameter distributions, but also the prior predictive distributions with brms (Bürkner (2017)).

## Hierarchical loss reserving with growth curves using brms

Ahead of the Stan Workshop on Tuesday, here is another example of using brms (Bürkner (2017)) for claims reserving. This time I will use a model inspired by the 2012 paper A Bayesian Nonlinear Model for Forecasting Insurance Loss Payments (Zhang, Dukic, and Guszcza (2012)), which can be seen as a follow-up to Jim Guszcza’s Hierarchical Growth Curve Model (Guszcza (2008)). I discussed Jim’s model in an earlier post using Stan.

## Models are about what changes, and what doesn't

How do you build a model from first principles? Here is a step by step guide. Following on from last week’s post on Principled Bayesian Workflow I want to reflect on how to motivate a model. The purpose of most models is to understand change, and yet, considering what doesn’t change and should be kept constant can be equally important. I will go through a couple of models in this post to illustrate this idea.

## PK/PD reserving models

This is a follow-up post on hierarchical compartmental reserving models using PK/PD models. It will show how differential equations can be used with Stan/ brms and how correlation for the same group level terms can be modelled. PK/ PD is usually short for pharmacokinetic/ pharmacodynamic models, but as Eric Novik of Generable pointed out to me, it could also be short for Payment Kinetics/ Payment Dynamics Models in the insurance context.

## Hierarchical compartmental reserving models

Today, I will sketch out ideas from the Hierarchical Compartmental Models for Loss Reserving paper by Jake Morris, which was published in the summer of 2016 (Morris (2016)). Jake’s model is inspired by PK/PD models (pharmacokinetic/pharmacodynamic models) used in the pharmaceutical industry to describe the time course of effect intensity in response to administration of a drug dose. The hierarchical compartmental model fits outstanding and paid claims simultaneously, combining ideas of Clark (2003), Quarg and Mack (2004), Miranda, Nielsen, and Verrall (2012), Guszcza (2008) and Zhang, Dukic, and Guszcza (2012).

## Changing settlement rate model for paid losses

Last week I wrote about Glenn Meyers’ correlated log-normal chain-ladder model (CCL), which he presented at the 10th Bayesian Mixer Meetup. Today, I will continue with a variant Glenn also discussed: The changing settlement log-normal chain-ladder model (CSR). Glenn used the correlated log-normal chain-ladder model on reported incurred claims data to predict future developments. However, when looking at paid claims data, Glenn suggested to change the model slightly. Instead allowing for correlation across accident years, he allows for a gradual shift in the payout pattern to account for a change in the claim settlement rate across accident years.

On 23 November Glenn Meyers gave a fascinating talk about The Bayesian Revolution in Stochastic Loss Reserving at the 10th Bayesian Mixer Meetup in London. Glenn worked for many years as a research actuary at Verisk/ ISO, he helped to set up the CAS Loss Reserve Database and published a monograph on Stochastic loss reserving using Bayesian MCMC models. In this blog post I will go through the Correlated Log-normal Chain-Ladder Model from his presentation.

## Notes from 4th Bayesian Mixer Meetup

Last Tuesday we got together for the 4th Bayesian Mixer Meetup. Product Madnesskindly hosted us at their offices in Euston Square. About 50 Bayesians came along; the biggest turn up thus far, including developers of PyMC3(Peadar Coyle) and Stan(Michael Betancourt).The agenda had two feature talks by Dominic Steinitzand Volodymyr Kazantsevand a lightning talk by Jon Sedar.Dominic Steinitz: Hamiltonian and Sequential MC samplers to model ecosystemsDominic shared with us his experience of using Hamiltonian and Sequential Monte Carlo samplers to model ecosystems.

## Fitting a distribution in Stan from scratch

Last week the French National Institute of Health and Medical Research (Inserm) organised with the Stan Groupa training programme on Bayesian Inference with Stan for Pharmacometricsin Paris. Daniel Leeand Michael Betancourt, who run the course over three days, are not only members of Stan's development team, but also excellent teachers. Both were supported by Eric Novik, who gave an Introduction to Stanat the Paris Dataiku User Grouplast week as well.Eric Kramer (Dataiku), Daniel Lee, Eric Novik & Michael Betancourt (Stan Group)I have been playing around with Stan on and off for some time, but as Eric pointed out to me, Stan is not that kind of girl(boy?

## Notes from 3rd and 3.5th Bayesian Mixer Meetup

Two Bayesian Mixer meet-ups in a row. Can it get any better?Our third ‘regular’ meeting took place at Cass Business School on 24 June. Big thanks to Pietroand Andreas, who supported us from Cass. The next day, Jon Sedar of Applied AI, managed to arrange a special summer PyMC3 event.3rd Bayesian Mixer meet-upFirst up was Luis Usier, who talked about cross validation. Luis is a former student of Andrew Gelman, so, of course, his talk touched on Stan and the ‘loo’ (leave one out) package in R.

## Notes from 2nd Bayesian Mixer Meetup

Last Friday the 2nd Bayesian Mixer Meetup (@BayesianMixer) took place at Cass Business School, thanks to Pietro Millossovichand Andreas Tsanakas, who helped to organise the event.Bayesian Mixer at CassFirst up was Davide De March talking about the challenges in biochemistry experimentation, which are often characterised by complex and emerging relations among components. The very little prior knowledge about complex molecules bindings left a fertile field for a probabilistic graphical model. In particular, Bayesian networks can help the investigator in the definition of a conditional dependence/independence structure where a joint multivariate probability distribution is determined.

## Bayesian Mixer on Meetup

We had our first successful Bayesian Mixer Meetuplast Friday night at the Artillery Arms!We expected about 15 - 20 people to turn up, when we booked the function room overlooking Bunhill Cemetery and Bayes’ grave. Now, looking at the photos taken during the evening, it seems that our prior believe was pretty good.The event started with a talk from my side about some very basic Bayesian models, which I used a while back to get my head around the concepts in an insurance context.

## First Bayesian Mixer Meeting in London

There is a nice pub between Bunhill Fields and the Royal Statistical Society in London: The Artillery Arms. Clearly, the perfect place to bring people together to talk about Bayesian Statistics. Well, that’s what Jon Sedar (@jonsedar, applied.ai) and I thought. Source: http://www.artillery-arms.co.uk/Hence, we’d like to organise a Bayesian Mixer Meetup on Friday, 12 February, 19:00. We booked the upstairs function room at the Artillery Arms and if you look outside the window, you can see Thomas Bayes’ grave.

## Loss Developments via Growth Curves and Stan

Last week I posted a biological example of fitting a non-linear growth curvewith Stan/RStan. Today, I want to apply a similar approach to insurance data using ideas by David Clark [1]and James Guszcza [2].Instead of predicting the growth of dugongs (sea cows), I would like to predict the growth of cumulative insurance loss payments over time, originated from different origin years. Loss payments of younger accident years are just like a new generation of dugongs, they will be small in size initially, grow as they get older, until the losses are fully settled.

## Bayesian regression models using Stan in R

It seems the summer is coming to end in London, so I shall take a final look at my ice cream data that I have been playing around with to predict sales statistics based on temperature for the last couple of weeks [1], [2], [3].Here I will use the new brms(GitHub, CRAN) package by Paul-Christian Bürknerto derive the 95% prediction credible interval for the four models I introduced in my first post about generalised linear models.

## Posterior predictive output with Stan

I continue my Stan experimentswith another insurance example. Here I am particular interested in the posterior predictive distribution from only three data points. Or, to put it differently I have a customer of three years and I'd like to predict the expected claims cost for the next year to set or adjust the premium.The example is taken from section 16.17 in Loss Models: From Data to Decisions[1]. Some time ago I used the same example to get my head around a Bayesian credibility model.

## Hello Stan!

In my previous postI discussed how Longley-Cook, an actuary at an insurance company in the 1950's, used Bayesian reasoning to estimate the probability for a mid-air collision of two planes.Here I will use the same model to get started with Stan/RStan, a probabilistic programming language for Bayesian inference. Last week my prior was given as a Beta distribution with parameters $\alpha=1, \beta=1$ and the likelihood was assumed to be a Bernoulli distribution with parameter $\theta$: \begin{aligned}\theta & \sim \mbox{Beta}(1, 1)\\\y_i & \sim \mbox{Bernoulli}(\theta), ;\forall i \in N\end{aligned}For the previous five years no mid-air collision were observed, $x={0, 0, 0, 0, 0}$.

## Predicting events, when they haven't happened yet

Suppose you have to predict the probabilities of events which haven't happened yet. How do you do this?Here is an example from the 1950s when Longley-Cook, an actuary at an insurance company, was asked to price the risk for a mid-air collision of two planes, an event which as far as he knew hadn't happened before. The civilian airline industry was still very young, but rapidly growing and all Longely-Cook knew was that there were no collisions in the previous 5 years [1].

## Extended Kalman filter example in R

Last week's post about the Kalman filterfocused on the derivation of the algorithm. Today I will continue with the extended Kalman filter (EKF)that can deal also with nonlinearities. According to Wikipedia the EKF has been considered the de facto standard in the theory of nonlinear state estimation, navigation systems and GPS.Kalman filterI had the following dynamic linear model for the Kalman filter last week:\begin{align}x_{t+1} & = A x_t + w_t,\quad w_t \sim N(0,Q)\\\y_t &=G x_t + \nu_t, \quad \nu_t \sim N(0,R)\\\x_1 & \sim N(\hat{x}_0, \Sigma_0)\end{align}With $x_t$ describing the state space evolution, $y_t$ the observations, $A, Q, G, R, \Sigma_0$ matrices of appropriate dimensions, $w_t$ the evolution error and $\nu_t$ the observation error.

## Kalman filter example visualised with R

At the last Cologne R user meetingHolger Zien gave a great introduction to dynamic linear models(dlm). One special case of a dlm is the Kalman filter, which I will discuss in this post in more detail. I kind of used it earlier when I measured the temperature with my Arduinoat home. Over the last week I came across the wonderful quantitative economic modelling site quant-econ.net, designed and written by Thomas J.

## Notes from the Kölner R meeting, 12 December 2014

Last week's Cologne R user group meeting was the best attended so far, and it was a remarkable event - I believe not a single line of R code was shown. Still, it was an R user group meeting with two excellent talks, and you will understand shortly why not much R code needed to be displayed.Introduction to Julia for R UsersDownload slidesHans Werner Borchers joined us from Mannheim to give an introduction to Juliafor R users.

## Measuring temperature with my Arduino

It is really getting colder in London - it is now about 5°C outside. The heating is on and I have got better at measuring the temperature at home as well. Or, so I believe.Last week's approachof me guessing/feeling the temperature combined with an old thermometer was perhaps too simplistic and too unreliable. This week's attempt to measure the temperature with my Arduino might be a little OTT (over the top), but at least I am using the micro-controller again.

## How cold is it? A Bayesian attempt to measure temperature

It is getting colder in London, yet it is still quite mild considering that it is late November. Well, indoors it still feels like 20°C (68°F) to me, but I have been told last week that I should switch on the heating. Luckily I found an old thermometer to check. The thermometer showed 18°C. Is it really below 20°C? The thermometer is quite old and I'm not sure that is works properly anymore.

## Hit and run. Think Bayes!

At the R in Insurance conferenceArthur Charpentiergave a great keynote talk on Bayesian modelling in R. Bayes’ theorem on conditional probabilities is strikingly simple, yet incredibly thought provoking. Here is an example from Daniel Kahnemanto test your intuition. But first I have to start with Bayes’ theorem.Bayes’ theoremBayes’ theorem states that given two events $D$ and $H$, the probability of $D$ and $H$ happening at the same time is the same as the probability of $D$ occurring, given $H$, weighted by the probability that $H$ occurs; or the other way round.

## Binomial testing with buttered toast

Rasmus’ post of last week on binomial testingmade me think about p-values and testing again. In my head I was tossing coins, thinking about gender diversity and toast. The toast and tossing a buttered toast in particular was the most helpful thought experiment, as I didn't have a fixed opinion on the probabilities for a toast to land on either side. I have yet to carry out some real experiments.

## Not only verbs but also believes can be conjugated

Following on from last week, where I presented a simple example of a Bayesian network with discrete probabilitiesto predict the number of claims for a motor insurance customer, I will look at continuous probability distributions today. Here I follow example 16.17 in Loss Models: From Data to Decisions [1]. Suppose there is a class of risks that incurs random losses following an exponential distribution (density $f(x) = \Theta {e}^{- \Theta x}$) with mean $1/\Theta$.

## Predicting claims with a Bayesian network

Here is a little Bayesian Network to predict the claims for two different types of drivers over the next year, see also example 16.15 in [1]. Let's assume there are goodand baddrivers. The probabilities that a good driver will have 0, 1 or 2 claims in any given year are set to 70%, 20% and 10%, while for bad drivers the probabilities are 50%, 30% and 20% respectively. Further I assume that 75% of all drivers are good drivers and only 25% would be classified as bad drivers.