Sunday, April 22, 2007

Wiki on research methods

A while ago, I wrote a book called 'Research methods and statistics in psychology', which was aimed at first year students studying psychology at university in the UK.

I've now made it into a Wiki (well, the text, anyway). Anyone is free to read it, and edit it. You can find it at

By the nature of Wikis, it's free as in beer, and free as in speech. I wonder if it's an interesting idea to let students write or change a textbook. Maybe they won't. I'm wondering how much of a role I should / will play. Those 50,000 (or so) words were my baby. Will I get upset if people mash them around? As it says on every editing page "If you don't want your writing to be edited mercilessly, then don't submit it here." Will I have urges to correct things I believe to be wrong? Will it get too big for me to keep track of whether things are wrong? (I doubt it, but you never know).

It's got Google ads on it, which means that it might make me back the $6/month that I spend on hosting. But I suspect it won't quite.

Monday, April 02, 2007

Book page updated

I've updated the page of supplemental information for the Understanding and Using Statistics in Psychology page. You can see it here, but it's not much use if you haven't got the book.

Sunday, April 01, 2007

What's the Denominator

When you want to try to fiddle people with statistics, to make yourself look better, one thing you can do is to try to change the denominator in your equation. (The denominator is the underneath number of a fraction, so if we say 3/4, the numerator is 3, the denominator is 4).

There was a story in today's Guardian that gives a nice example of this. The story was about ticket touting, and whether Ebay is used by touts. Here's the quote:
Ebay disputes that large-scale ticket touting takes place on its site, citing research by ICM last year that showed nine out of 10 eBay sellers who had listed tickets in the previous 12 months had listed less than five.
(We'll ignore the fact that they should have said 'fewer than', not less than.)

They are using sellers as the denominator, but that's not the right one. The appropriate denominator is tickets. Let's imagine that we had 10 sellers. 9 of those 10 sellers sold 4 tickets (making 36 tickets). The tenth seller sold 964 tickets. This matches what Ebay said, but it's not relevant. What is relevant is that 96.4% of tickets are sold by people who sell more than 500 per year. That looks like a problem.

It isn't necessarily the case that this is true - but it's not ruled out by what they say. Lies, damn lies and statistics may be true - but the lies aren't very hard to see through.


Originally uploaded by Beautiful Freaks.
These are my twin sons, Alex (on the right) and Daniel (left), aged 4, reading a book called Polytomous Item Response Theory Models (click on the picture to make it larger).

If I saw this picture, I'd think it was posed, but I really did find them like this.