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.

About half a year ago Ian Branagan, Chief Risk Officer of Renaissance Re - a Bermudian reinsurance company with a focus on property catastrophe insurance, gave a talk about the usage of models in risk management and how they evolved over the last twenty years. Ian’s presentation, titled with the famous quote of George E.P. Box: “All models are wrong, but some are useful”, was part of the lunch time lecture series of talks at Lloyd’s, organised by the Insurance Institute of London.

I was trained as a mathematician and it was only last year, when I attended the Royal Statistical Society conference and met many statisticians that I understood how different the two groups are.
In mathematics you often start with some axioms, things you assume to be true, and these axioms are then the basis from which new theory is derived. In statistics or more general in science you start with a theory, or better a hypothesis and try to disprove it.

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