Stats Chat

Each week, we would like to invite readers of Stats Chat to submit nominations for our Stat of the Week competition and be in with the chance to win an iTunes voucher.

Here’s how it works:

• Anyone may add a comment on this post to nominate their Stat of the Week candidate before midday Friday September 23 2011.
• Statistics can be bad, exemplary or fascinating.
• The statistic must be in the NZ media during the period of September 17-23 2011 inclusive.
• Quote the statistic, when and where it was published and tell us why it should be our Stat of the Week.

Next Monday at midday we’ll announce the winner of this week’s Stat of the Week competition, and start a new one.

The fine print:

• Judging will be conducted by the blog moderator in liaison with staff at the Department of Statistics, The University of Auckland.
• The judges’ decision will be final.
• The judges can decide not to award a prize if they do not believe a suitable statistic has been posted in the preceeding week.
• Only the first nomination of any individual example of a statistic used in the NZ media will qualify for the competition.
• Employees (other than student employees) of the Statistics department at the University of Auckland are not eligible to win.
• The person posting the winning entry will receive a $20 iTunes voucher.
• The blog moderator will contact the winner via their notified email address and advise the details of the $20 iTunes voucher to that same email address.
• The competition will commence Monday 8 August 2011 and continue until cancellation is notified on the blog.

No winner this week due to a lack of nominations – please add your suggestions for this week’s competition!

The factoid of dramatically increasing cannabis potency has popped up again, with a claim that cannabis used to be 1-2% THC and is now up to 33%.    The most comprehensive and consistent data on cannabis potency come from a long-term project at the University of Mississippi. Their 2010 paper is based on analysis of 46,000 confiscated samples from 1993 to 2008.    Over this time period, the percentage of THC in marijuana (leaves and buds with seeds) increased from about 3.5% to about 6%.  The percentage in sinsemilla (buds without seeds) increased from about 6% to about 11%.   Since the more-recent samples were more likely to be sinsemilla, the percentage over all confiscated samples increased a bit more, from about 3.5% to about 9%.  A small fraction of the samples had much higher concentrations, but this fraction didn’t change much over time. So, yes, the average used to be about 3% in 1993 and may have been as low as 2% in earlier decades, and, yes, the concentration is now ‘up to‘ 33%, but the trend is nothing like as strong as that suggests.   A New Zealand paper , by ESR researchers (who are hardly pot-sympathising hippies), says that there was no real change in THC concentration in cannabis plant material from 1976 to 1996, and the concentration in cannabis oil actually fell.

The Southland Times article also reports a claim that 90% of first-term methamphetamine users continue to use the drug. If this just means that 90% of them go on to have a second dose at some time it might well be true, but if it is implying long-term addiction the figure seems implausible. It’s certainly not what is found in other countries.  For example, the most recent results from the US National Survey on Drug Use and Health (NSDUH) estimate that 364000 people in the US had dependence/abuse of illegal stimulants in 2010. If we assume that all of these were methamphetamine, and that the other illegal stimulants didn’t cause any dependence/abuse problems, that’s still only 20% of the estimated 1.8 million people who first tried methamphetamine in the period 2002-2010. In fact, since NSDUH has a nice online table generator we can do a more specialized query and find out that an estimated 118000 people currently had dependence on stimulants out of the estimated 10 million people who had ever tried methamphetamine. That’s more like 1% than 90%.   Amphetamines are clearly something you want to stay well away from, but there’s no way that they addict 90% of the people who try them. In any case, if we believe the drug warriors, New Zealand’s P epidemic has already been solved by banning pseudoephedrine without a prescription.

I’m all for getting teenagers to appreciate the risks of drug use, but we need to remember teenagers can use Google too.

Last week’s Stat of the Week nomination was a story on the “J-curve” for disease risk and alcohol consumption.  Yet another research paper, this time from the Nurses’ Health Study, had found that people who drink small amounts of alcohol regularly are healthier than those who drink none and those who drink larger amounts.    This sort of result is unpopular with doctors, as the Herald story reported, and for good reasons, but that doesn’t mean it’s untrue.  On the other hand, the fact that it’s true doesn’t mean that it’s news.

The obvious difficulty in comparing drinkers to non-drinkers is that some of the strictest non-drinkers are actually ex-drinkers, people who you would expect to be in worse health.  Since epidemiologists are not completely stupid, they know about this problem and many studies have addressed it. Excluding ex-drinkers doesn’t make the effect go away, waiting for a long time between the drinking assessment and the health assessment (as in this paper) doesn’t make it go away, and splitting up light drinking into finer categories shows that there is lower risk for people who drink occasionally than for those who regularly drink a small amount (again, as in this paper).  For some of the claimed benefit there are even plausible mechanisms (eg, alcohol consumption does definitely raise HDL cholesterol levels in short-term experimental studies).   This is just observational research, so the results could be just as wrong as the apparent protective effect of beta-carotene in cancer, or of raising HDL cholesterol with niacin in heart disease, which fell apart when subjected to randomised trials, but it’s carefully-done observational research.

As doctors will tell you, the problem with announcing a health benefit of moderate alcohol consumption is that most people interpret “moderate” to mean “a bit more than I currently drink”.  As a scientific result, it’s fine; as a public-health intervention, it’s badly off-target.   The American Heart Association guidelines on alcohol and heart disease, for example, basically say that regular consumption of small amounts of alcohol probably is protective against heart disease, but that you shouldn’t go around advocating it.

The problem for medical researchers is that funding bodies and universities (and their own egos) want press coverage of research results, but that this sort of marketing of incremental medical research as if it was ground-breaking health advice is unhelpful to the public. It’s very rare that you should change your behavior based on the results of a single medical study, but that’s the model that a lot of medical reporting is based around.

Congratulations to Eric Crampton for another great nomination and we have awarded it as our winner. It’s a fairly technical, well-researched Stat of the Week which is argued well.

Each week, we would like to invite readers of Stats Chat to submit nominations for our Stat of the Week competition and be in with the chance to win an iTunes voucher.

Here’s how it works:

• Anyone may add a comment on this post to nominate their Stat of the Week candidate before midday Friday September 16 2011.
• Statistics can be bad, exemplary or fascinating.
• The statistic must be in the NZ media during the period of September 10-16 2011 inclusive.
• Quote the statistic, when and where it was published and tell us why it should be our Stat of the Week.

Next Monday at midday we’ll announce the winner of this week’s Stat of the Week competition, and start a new one.

The fine print:

• Judging will be conducted by the blog moderator in liaison with staff at the Department of Statistics, The University of Auckland.
• The judges’ decision will be final.
• The judges can decide not to award a prize if they do not believe a suitable statistic has been posted in the preceeding week.
• Only the first nomination of any individual example of a statistic used in the NZ media will qualify for the competition.
• Employees (other than student employees) of the Statistics department at the University of Auckland are not eligible to win.
• The person posting the winning entry will receive a $20 iTunes voucher.
• The blog moderator will contact the winner via their notified email address and advise the details of the $20 iTunes voucher to that same email address.
• The competition will commence Monday 8 August 2011 and continue until cancellation is notified on the blog.

Expect to hear a lot of big numbers thrown about during the Rugby World Cup – and not just about those on the field.

When Auckland hosts the Rugby World Cup Opening Night Celebrations this Friday, people will want to know how many turned up. The success of the event (and justification for the expenditure) will be measured in part by estimated crowd size.

Crowd estimation is often not at all scientific. During tonight’s ONE News bulletin, reporter Jack Tame estimated there were “five or ten thousand delirious Tongan fans”.

How are crowd estimate figures obtained and how reliable are they?

The authors of a new study published in Significance, the magazine of the Royal Statistical Society and the American Statistical Association claim that most crowd estimations are unreliable and that the public should view crowd estimation with scepticism:

“In the absence of any accurate estimation methods, the public are left with a view of the truth coloured by the beliefs of the people making the estimates,” claims Professor Paul Yip, of the University of Hong Kong, one of the authors of the study.

“It is important to rectify the myth of counting people. The public would be better served by estimates less open to political bias. Our study shows that crowd estimates with a margin of error of less than 10% can be achieved with the proposed method.”

Further reading:

• How many were there when it mattered?
• Feeding the Five Thousand – or was it Three?
• The Protest Numbers Game

Also of interest:

• Royal Wedding TV audience closer to 300m than 2bn

Guest post by Tony Cooper

Paul The Octopus is famous for picking the winner in 8 out of 8 games at the FIFA World Cup in 2010. How did he achieve this amazing feat? Was he skilled or was he lucky?

To get 8 out of 8 games right where each game is a 50-50 guess the probability is 0.5 x 0.5 x 0.5 x 0.5 x 0.5 x 0.5 x 0.5 x 0.5 = 0.0039 (or one chance in 256). This seems too incredible. An octopus can’t be that good.

Where is the flaw in this probabilistic reasoning?

The answer is that the probability that Paul got a game right was not 0.5 but more. Much more. In fact the probability was much closer to one. Here is the explanation:

Paul, a German octopus, was only used to pick German games, usually picked the German team, and the German team usually won. So the chance that Paul picked the winner was more than 0.5 for each game. That accounts for 5 of the 8 games.

What about the games that Germany lost and the game where Germany didn’t play? Let’s take a guess.

This tournament was not Paul’s first time at playing the game. He had learned previously that there was always food under the German flag. Paul – with some ability to distinguish the flags of different countries – usually went for the German black, red, and yellow. He might have had monochrome eyesight which is why he picked Serbia as a winner in the game against Germany because in monochrome the Serbian and German flags look similar.

For the final Germany wasn’t playing so Paul went for the most German looking flag – that of Spain (red, yellow, red). He seems to have a preference for that flag since he also predicted the win of Spain over Germany in the semi-finals. All flags picked by Paul had horizontal stripes.

So Paul was lucky but not as lucky as the 1 in 256 chance suggests. His main luck was that his team was one of the best teams in the tournament and that some of the other good teams had similar flags.

Will Richie McCow be successful at picking the winner of the All Black games in the Rugby World Cup? Possibly – as long as he keeps picking the All Blacks and the All Blacks keep winning. Will the All Blacks keep winning? That’s a story for a later article.

Tony Cooper, formerly of the Applied Mathematics Division of the DSIR, is a Quantitative Analyst with Double-Digit Numerics of Auckland. He consults mainly in the investment, finance, and electricity industries. His research interests include risk and volatility prediction, alpha generation, data mining, statistical learning, and time series analysis.

Thank you for the three nominations for this week’s Stat of the Week competition.

Tony’s nomination of Spirit Level has been highly politicized and would require us to be ready to provide detailed rebuttals to the proponents of both sides of the argument. Life is too short!

We’d also like to hear the results of Ian Well’s complaint to the NZBCA of Growpro’s statistics and find out more details before commenting on them. (Ian, please advise us when the ruling comes out if there are comments on the statistics.)

Miranda Devlin’s nomination makes a good point that the obesity and overweight figures quoted are overall ones, rather than just for women (as the whole article is about them). However, in the sentence before these statistics are mentioned, the article does point out the obesity percentage for women.

In saying all this, we aren’t selecting a Stat of the Week this week but encourage you to discuss the nominations further if you wish.

Each week, we would like to invite readers of Stats Chat to submit nominations for our Stat of the Week competition and be in with the chance to win an iTunes voucher.

Here’s how it works:

• Anyone may add a comment on this post to nominate their Stat of the Week candidate before midday Friday September 9 2011.
• Statistics can be bad, exemplary or fascinating.
• The statistic must be in the NZ media during the period of September 3-9 2011 inclusive.
• Quote the statistic, when and where it was published and tell us why it should be our Stat of the Week.

Next Monday at midday we’ll announce the winner of this week’s Stat of the Week competition, and start a new one.

The fine print:

• Judging will be conducted by the blog moderator in liaison with staff at the Department of Statistics, The University of Auckland.
• The judges’ decision will be final.
• The judges can decide not to award a prize if they do not believe a suitable statistic has been posted in the preceeding week.
• Only the first nomination of any individual example of a statistic used in the NZ media will qualify for the competition.
• Employees (other than student employees) of the Statistics department at the University of Auckland are not eligible to win.
• The person posting the winning entry will receive a $20 iTunes voucher.
• The blog moderator will contact the winner via their notified email address and advise the details of the $20 iTunes voucher to that same email address.
• The competition will commence Monday 8 August 2011 and continue until cancellation is notified on the blog.