Independence, independence, independence

Multiplying probabilities without any thought is a dangerous game.  Just look at this piece in Rolling Stone, referenced in this Guardian article.

...warmest May on record for the Northern Hemisphere – the 327th consecutive month in which the temperature of the entire globe exceeded the 20th-century average, the odds of which occurring by simple chance were $3.7 \times 10^{-99}$, a number considerably larger than the number of stars in the universe.

Horse Racing and Coincidence

Just penned this to the BBC about why their piece on a jockey winning seven races in a single day quotes the wrong odds.

In your article on Richard Hughes winning seven races in a single day, you quote the odds of this event as being 10,168-1.  Whilst undoubtedly a fantastic achievement, these odds are incorrect, since they they ignore the fact that Mr Hughes raced in eight races that day.  The chances of him winning 7 out of 8 races (or more) is about 1,257-1, which is a bit more modest.  In particular, it seems fairly unlikely that someone would place a bet on the rider winning these particular seven races, and not the eighth.

Fruit, Vegetables, Health and Happiness

Whilst perusing the Daily Mail today (my excuse is that it's next to the espresso machine) I saw this interesting health advice:

Forget five a day: Now scientists say you'll be healthier and happier eating seven daily portions of fruit and veg.

The phrase "scientists say" is always a red light. 

Trust your instincts

As Obi Wan almost said: "Let go your conscious self and act on instinct.  Your news articles, government agencies and other organisations can deceive you, don't trust them."

My brother Andrew (@andysstudy) has been studiously following this advice since 1977, and sent me this tweet:

See below. There are 73m children (u15) in EU I find it hard to believe 1:73 goes missing every year? http://www.euronews.com/2012/10/01/has-anyone-seen-my-child/

Road safety - how not to reason from data

My apologies for going quiet over the summer.  I've been in China, and they're not too big on blogs there (blogger is blocked).

Just a short post here, on something fairly obvious.  Here's an extract from post number 51 by Cambridge News' Cycling Blog on speed limits.

In Great Britian in 2011, 7 people were killed on a road with a 20 mph limit. 636 killed in a 30 mph limit. 289 people were seriously injured in a 20 mph limit, 13,168 in a 30 mph limit... Yes, these are large numbers. But it is the proportions that matter here... So that is proof then. Lower speed limits means fewer people killed.

[In case you're wondering, there's no irony in the last sentence.  At all.]  I assume you're all thinking the same as me by now.

Polygraphs and Sex Offenders

Just a short post on the news story that a pilot scheme which administers polygraph tests ('lie detector tests') to sex offenders on probation has been deemed successful, and the Ministry of Justice plans to roll it out nationwide.

The MoJ's research seems reasonable enough: the pilot scheme took place in the Midlands, and comparison groups of offenders were selected from other areas.  The assignment was not randomised, which is unfortunately all too common, but the comparison group was similar to the treatment group on the basis of the most obvious covariates (age, original offence, risk or reoffending, criminal history, etc.).  Based on a quick-ish reading of the paper it seems fairly solid.

Reading too much into one number

The BBC points out that deaths from road accidents in the UK increased last year for the first time in 10 years.  In total 1,901 people were killed during 2011, which is 51 more than in 2010.  This is certainly not good.

But is it surprising, in a statistical sense?

Sigma-tised

Is it just me, or is everyone completely unable to explain what a confidence interval is?

The Higgs Boson is back in the news again, here's a Nature News article discussing Cern's latest discovery, which is that

The data contained “a significant excess" of collision events at a mass of around 125 gigaelectronvolts...

 Physicists have maintained that they will not announce the discovery of the Higgs until the signal surpasses 5 sigma, meaning that it has just a 0.00006% chance of being wrong. The ATLAS and CMS experiments are each seeing signals between 4.5 and 5 sigma, just a whisker away from a solid discovery claim.

This is wrong.

Data Overload?

Today I guest blogged about the government's open data plans on the Understanding Uncertainty website.  Check it out here.

National Traits and the Ecological Fallacy

This article caught my eye, headlined "Why criminals believe in heaven." [The Daily Mail's 'Science' and 'Health' pages are an unlimited resource for writing blog posts about bad statistics, but I promise to try not to only pick on them in future.]

The original paper is here (predictably the DM don't bother to link to it), and here is the university's press release which has been pretty slavishly copied to create the article.   The summary is, the researchers combined crime data from one source (the United Nations Office on Drugs and Crime), with survey data on people's attitudes to religion and other things (from World Values Surveys and European Value Surveys).  They found that the belief in heaven and hell is a strong predictor of crime rates at a country-wide level, whilst rates of particular religious beliefs were not.

Review: The Geek Manifesto

This week I've been reading Mark Henderson's book The Geek Manifesto: Why Science Matters, which discusses the role of science and its geeky proponents in public life. Henderson's principal thesis is that politicians and scientists have been wilfully ignorant of each other for too long, and that it is time for science, its methods, and its practitioners to take a more central place in politics and policy.  This includes statistics, of course, which is why it seemed relevant to the blog.

How should statistics be taught? Some thoughts.

Inspired by Timothy Gowers' recent post on how mathematics should be taught to non-mathematicians, I thought it might be prudent to ask how statistics should be taught.  If you need to be motivated as to how important teaching statistics is, watch Arthur Benjamin's short TED talk.

Newsflash: everyone being healthier reduces deaths

People like me often complain about the accuracy of scientific journalism, and the selective nature of the reports we see in the media; indeed, there's plenty to complain about.  However it's perfectly possible to misrepresent a story simply by choosing to place a particular spin upon it, usually one which makes the story seem more political than the original research.  It's easy to blame the newspapers for this, but often the problem is compounded by academic press releases, which are themselves designed to catch the eye of media outlets.

What is probability?

I haven't seen any particularly interesting (or unusually egregious) uses of statistics in the last few days, so it's time to have a more philosophical outing.  Of course, if you see anything in the news you think is worthy of a post, email me (evans [at] stats [dot] ox [dot] ac [dot] uk).

So to the title of the post - what is probability?  Probability can be viewed as just a mathematical construct, which we'll explore some other time, but even with the mathematical rules in place, there are various interpretations of what a probability actually is.  At school we usually think about rolling dice and flipping coins, so let's start there.  I flip a 50p coin: what's the probability that it comes up heads?  You'd probably [sorry] say it's 'evens', 'a half', 'fifty-fifty', or (as mathematicians generally prefer), '0.5'.

Fast food, dodgy infographics, implausible claims

The Daily Mail reported on Wednesday that US restaurant portion sizes had quadrupled(!) since the 1950s, a claim faithfully reproduced from the Center for Disease Control's (CDC) website.  The article comes complete with this infographic.

Now, it should be immediately obvious (if you remember your GCSE maths) that something is not quite right with this picture: the fountain soda looks truly enormous.  Taking the numbers in the diagram on faith for the moment (but see below), the volume of a cup has increased from 7 oz (200 ml) to 42 oz (1.25 l), a worrying factor of six increase.  In accordance with this, the 'NOW' cup on the diagram is 6 times taller than the 1950s cup.

Non-existent recidivism statistics wrongly reported

This article from the BBC News website caught my eye today, headlined "Reoffending rates reach record level".  It also says that 

Ministry of Justice officials say the figures show a "clear trend" of a rising re-offending rate.

The Independent makes some similar claims.  The phrase "re-offending rate" is simple enough: it's the proportion of people who, having been convicted of a crime, go on to commit another one within a year, or within five years, or whatever.  Right?