I had an interesting exchange by e-mail with Maggie Smith, the analyst at UCAS responsible for the note discussed in my previous blog entry. She tells me that the conclusion I derived from the released data (the small but significant ethnicity effect) is very much attenuated when the data is broken down by provider, and the published UCAS analysis is based on such disaggregated data. Continue reading Update on UCAS post
UCAS, ethnicity and admission rates
UCAS, the UK university admissions clearing house, have released data relating to ethnicity and admissions to English universities, in part in response to Vikki Boliver‘s research in Sociology suggesting that members of ethnic minorities are less likely to be admitted to Russell Group universities.
The analysis note with the release is sober and correct, showing a mostly consistent pattern of offer rates for ethnic minority students being lower (but not far lower) than expected. However, UCAS’s press release seems to have suggested that the effect is almost explained away, and attributes it to ethnic minority students disproportionately applying to courses with low acceptance rates. This does not seem to be the case.
Update: see also next blog entry.
Continue reading UCAS, ethnicity and admission rates
Multi-processor Stata without Stata-MP
Exploit your cores!
If you don’t have Stata-MP, it can be difficult to benefit from all the cores on your computer. However, if your problem can be split up in parts that can run in parallel, it is easy to run multiple instances of Stata. In this note I demonstrate a simple case, using the example of a simulation I wish to run many times.
Substitution costs from transition rates
Given that determining substitution costs in sequence analysis is such a bone of contention, many researchers look for a way for the data to generate the costs. The typical way to do this is, is by pooling transition rates and defining the substitution cost to be:
2 – p(ij) – p(ji)
where p(ij) is the transition rate from state i to state j. Intuitively, states that are closer to each other will have higher transitions, and vice versa. Continue reading Substitution costs from transition rates
New Sequence Analysis Tools
I last released SADI, my sequence analysis tools for Stata, in November 2011. Since then I’ve made various improvements and additions, relating to ongoing work such as that reported in Dept Working Paper WP2012-02 and WP2013-05 (the latter is an early version of a paper that is coming out in the book of the LaCOSA conference, due shortly).
Continue reading New Sequence Analysis Tools
Hardline materialism in the Irish Times letter page
The text of my letter published in the Irish Times today (at http://www.irishtimes.com/debate/letters/philosophy-and-science-1.1667425):
Sir, – William Reville (Science, January 16th) criticises materialism as excluding, without evidence, the possibility of the supernatural. Continue reading Hardline materialism in the Irish Times letter page
Using Emacs to send mail later
There are lots of ways to schedule mail to be sent some time in the future, but it is easy, for those of us who write and send mail from Emacs, to use that program and the Unix atd batch system to do it. If you use message-mode to write messages, this approach means that creating mails for delayed sending is the same as for normal sending.
Mapping with Python and Stata
Elevation data for large swathes of the planet have been collected by NASA and are available to download from http://dds.cr.usgs.gov/srtm/.
The data is contained in binary files, each representing a 1-degree by 1-degree “square”. Here are five lines of Python and four lines of Stata that will turn the data into a simple graph:
import struct
file = open("data/N52W011.hgt", "r")
for y in range(1201):
for x in range(1201):
print y, x, struct.unpack(">h",file.read(2))[0]
Do python file.py > map.dat. Then run this Stata code:
infile i j height using /tmp/ext.dat
gen h2 = int(sqrt(height))
replace h2 = 30 if h2<=0
hmap j i h2, nosc
(Hi-res version.)
You may need to install Python’s struct package, and Stata’s hmap add on, but they’re available from the usual locations.
There are better ways of doing this, of course: it’s slow, the aspect ratio is wrong, the colours are not ideal and the axis labelling is bad. Even worse, it is a complete abuse of the hmap add-on. It’s a quick and dirty way to turn binary data into pictures, all the same.
Hedstrom’s Desires-Believes-Acts model in Emacs lisp
Emacs-lisp is a pretty functional language for managing Emacs and automating complex tasks within it, particularly to do with text processing. It’s probably not wise to use it for more general programming or analytical tasks, but every now and then (when I need to procrastinate, mostly) I get carried away.
A few years ago I was reading Peter Hedstrom’s book, Dissecting the Social, and realised his Desires-Believes-Acts model (a kind of cellular automaton) would be easy enough to implement. More recently, I noticed that Emacs’ tools for displaying simple games like Tetris (do “M-x tetris”) would permit a clean display.
In Hedstrom’s model, every cell in a grid may desire an outcome, and may believe they are able to achieve it. If they do both, they act. Belief and desire depend on the beliefs and desires of your neighbours. Generally, even starting from random and low distributions of belief and desire, within a number of iterations stable configurations emerge, with systematic segregation; often everyone acts in the end but sometime stable oscillating systems emerge.
Continue reading Hedstrom’s Desires-Believes-Acts model in Emacs lisp
Discrepancy analysis in Stata
In Studer et al (2011) an important new tool is introduced to the field of sequence analysis, the idea of “discrepancy” as a way of analysing pairwise distances. This quantity is shown to be analogous to variance, and is thus amenable to ANOVA-type analysis, which means it is a very attractive complement to cluster analysis of distance matrices.
This has been implemented in TraMineR (under R), along with a raft of other innovations coming out of Geneva and Lausanne. Up to now it hasn’t been available elsewhere. I spoke to Matthias Studer at the LaCOSA conference, and he convinced me that it was easy to code, and that all the information required was in the paper. This turned out to be the case, and I have written an initial Stata implementation. Continue reading Discrepancy analysis in Stata