Just a quick posting following up on the brms/rstanarm posting. In `brms 0.8`

, they’ve added non-linear regression. Non-linear regression is fraught with peril, and when venturing into that realm you have to worry about many more issues than with linear regression. It’s not unusual to hit roadblocks that prevent you from getting answers. (Read the Wikipedia links Non-linear regression and Non-linear least squares to get an idea.)

# R Users Will Now Inevitably Become Bayesians

There are several reasons why everyone isn’t using Bayesian methods for regression modeling. One reason is that Bayesian modeling requires more thought: you need pesky things like priors, and you can’t assume that if a procedure runs without throwing an error that the answers are valid. A second reason is that MCMC sampling — the bedrock of practical Bayesian modeling — can be slow compared to closed-form or MLE procedures. A third reason is that existing Bayesian solutions have either been highly-specialized (and thus inflexible), or have required knowing how to use a generalized tool like BUGS, JAGS, or Stan. This third reason has recently been shattered in the R world by not one but two packages: `brms`

and `rstanarm`

. Interestingly, both of these packages are elegant front ends to Stan, via `rstan`

and `shinystan`

.

This article describes `brms`

and `rstanarm`

, how they help you, and how they differ.

# Map Projections

The Earth is round, and maps are flat. That’s a problem for map makers. And a source of endless entertainment for geeks.

Carlos A. Furuti has an excellent website with many projections and clear explanations of the tradeoffs of each. The main projection page has links to all types, including two of my favorites: Other Interesting Projections, and Projections on 3D Polyhedra. Enjoy!

In R, the packages maps and mapproj are your entrĂ©e to this world. I created the above map (a Mollweide projection, which is a useful favorite), with:

`library (maps)`

library (mapproj)

```
```

`map ("world", projection="mollweide", regions="", wrap=TRUE, fill=TRUE, col="green")`

map.grid (labels=FALSE, nx=36, ny=18)

# Real Data Science pt1: Review of Numbersense

So far, when I’ve written on Data Science topics I’ve written about the fun part: the statistical analysis, graphs, conclusions, insights, etc. For this next series of postings, I’m going to concentrate more on what we can call *Real Data Science*®: the less glamorous side of the job, where you have to beat your data and software into submission, where you don’t have access to the tools or data you need, and so on. In other words, where you spend the vast majority of your time as a Data Scientist.

I’ll start the series with a review of Kaiser Fung’s Numbersense, published in 2013. It’s not mainly about Real Data Science, but I’ll start with it because it’s a great book that illustrate several common data pitfalls, and in the epilogue Kaiser shares one of his own Real Data Science stories and I found myself nodding my head and saying, “Yup, that’s how I spent several days in the last couple of weeks!”

# Dividing a sandwich fairly

I like to read various Stack Exchange websites, and one of them has a wonderful discussion of how you might divide a sandwich between three people fairly. Most of us are familiar with the two-person version: one person cuts and the other person gets the first choice. But what about if there are three people, or more?

Continue reading

# Book recommendation: Longitudinal Structural Equation Modeling

Longitudinal Structural Equation Modeling, Todd D. Little, Guilford Press 2013.

Let me start by saying that this is one of the best textbooks I’ve ever read. It was written as if the author was our mentor, and I really get the feeling that he’s sharing his wisdom with us rather than trying to be pedagogically correct. The book is full of insights on how he thinks about building and applying SEMs, and the lessons he’s learned the hard way.

# Free-form spreadsheet-ish app for Mac and Windows

I’ve just discovered a unique app on the Mac App Store called Calca. It’s like a simple word-processor, except you can define variables and functions and do arithmetic with them, and it understands units and currencies and it handles matrices and vectors, and supports basic Markdown, and … it’s pretty amazing.