Posts from October 2016 (37)

Q: Did you see that two cups of coffee a day will prevent dementia?

A: In Stuff? That’s not what it says.

Q: “Two cups of coffee a day can keep dementia at bay – research”

A: Read a bit further.

Q: Ok, so it’s just in women over 65, and two to three cups, and it’s a 36% reduction,  not as good as the headline says, but still pretty good, surely?

A: There’s a lot of uncertainty in that number

Q: So what’s the margin of error or whatever the medical folks call it?

A: According to the research paper a 95% confidence interval for the reduction goes from 1% to 44%. And it’s a reduction in rate, not in risk — it could easily be postponing rather than preventing dementia, even if it works.

Q: Was there a link to the paper?

A: No, but there was a link to the press release, and it linked to the paper.

Q: That interval. Why isn’t 36% in the middle of the interval?

A: I don’t know. The results in the abstract and tables of the paper give a hazard ratio of 0.74. I can think of two possibilities.  One is that the 36% isn’t based on the primary findings in the abstract but on a less well-described secondary analysis. The other is that someone subtracted 74% from 100% and got it wrong.

Q: Why is it just women over 65?

A: Because that’s who was in the study.

Q: So the coffee-drinking didn’t necessarily start at 65?

A: No, and it wasn’t necessarily coffee. It could have been tea or soda.

Q: Could they look at whether the coffee drinkers were different at the start of the study?

A: Yes — and they were.  The difference in their cognitive test scores stayed pretty much constant during the study, and the correlation with caffeine mostly goes away if you compare people starting out with the same test scores.

Q: So it might be that caffeine matters at an earlier age, not over 65?

A: And it might not matter — perhaps the people who drink a lot of caffeine were at lower risk for some other reason.

Q: Could it still be true?

A: It could.  It is in some lab-animal models of Alzheimer’s, but no-one really knows how relevant they are to human dementia

Q: Rats.

A: Yes, and mice.

Q: No, that was a colloquial exclamation expressing frustration, disappointment, or annoyance.

There’s a recent paper from Denmark finding that women, particularly young women, who used hormonal contraceptives were more likely also to be diagnosed with  depression.  The Guardian has a sensible story reporting on the paper (though given the topic it’s a pity the external experts they talked to were both men). There’s also an opinion piece, which conveys the importance of the issue but is clearly written by someone whose opinions were decided before the research came out. I was asked on Twitter what I thought.

One of the more difficult cases for science communication is where the evidence is neither negligible nor overwhelming, and that’s the situation here.  There’s nothing intrinsically unlikely about an effect on depression, and there are some ways that this study is very good, but there are also some limitations to the data that make the evidence weaker.

First, the good points. The study involved the entire Danish population over nearly 20 years, meaning that it was large enough to be fairly reliable on whether correlations are present or not, and also that it was comprehensive — it didn’t miss people out.  The data on who used hormonal contraceptives comes from the national health system and so should be accurate. The two definitions of depression — ‘prescribed anti-depressant drugs’ and ‘psychiatrist diagnosis of depression’ — will be measured reliably, and the decisions will have been taken by people who don’t have any particular view on the study question.  There’s information on timing, so we know the contraceptives were used before the depression. The associations are strong enough to care about, but not so strong as to be implausible. The analysis is well done given the data.

However, there are at least two alternative explanations for the correlation that aren’t ruled out by these data. The first is that the depression definitions require seeing a doctor and asking for (or at least accepting) treatment, and women who take hormonal contraceptives are probably more likely to see a doctor regularly.  The second explanation, which the researchers do consider, is that break-ups of relationships are a cause of depression, especially in younger people, and being in these relationships might be related to using hormonal contraceptives.  The researchers don’t believe this explanation, and they may be right, but their data don’t rule it out.

It’s not that either of these explanations is necessarily more likely than a direct effect of hormones, but if there weren’t alternative explanations the evidence would be stronger.  For example, if the researchers had been able to compare women using hormonal contraceptives just to those using non-hormonal contraceptives (eg copper IUDs and condoms), and had still seen the same correlation, the second explanation would be much less plausible and the evidence for a direct effect would be more convincing.

Also, if there were a straightforward hormonal explanation I would have expected different types of contraceptive to have stronger or weaker associations according to the dose of, say, progestins. In fact what they saw is that less commonly used contraceptives had stronger associations: weakest for the combined pills, stronger for progestin-only ‘mini-pills’ and stronger still for patch and implant methods. Again, this certainly doesn’t rule out a direct effect, but it weakens the evidence.

If a similar study were done in another country with different patterns of contraceptive use and found similar results, the evidence would become stronger. A study with fewer women but more detailed information on mental and emotional health — such as one of the birth-cohort studies — might be able to say more about what leads up to episodes of depression in young women and might be able to say something about who is at most risk. There’s still going to be uncertainty.

So. It’s hard to say for sure. There is definitely some evidence that hormonal contraceptives increase the risk of depression. If the effect is real, it’s useful to know that it seems to be largely in women under 20, largely in the first year of use, and might be worse for the ‘mini-pill’ than the traditional pill.  There’s a lot already known — good and bad — about hormonal contraceptives, but this research paper does add something.

Team Ratings for Round 8

The basic method is described on my Department home page.

Here are the team ratings prior to this week’s games, along with the ratings at the start of the season.

Performance So Far

So far there have been 54 matches played, 42 of which were correctly predicted, a success rate of 77.8%.
Here are the predictions for last week’s games.

Predictions for Round 8

Here are the predictions for Round 8. The prediction is my estimated expected points difference with a positive margin being a win to the home team, and a negative margin a win to the away team.

Team Ratings for Round 10

The basic method is described on my Department home page.

Here are the team ratings prior to this week’s games, along with the ratings at the start of the season.

Performance So Far

So far there have been 35 matches played, 26 of which were correctly predicted, a success rate of 74.3%.
Here are the predictions for last week’s games.

Predictions for Round 10

Here are the predictions for Round 10. The prediction is my estimated expected points difference with a positive margin being a win to the home team, and a negative margin a win to the away team.

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 October 7 2016.
• Statistics can be bad, exemplary or fascinating.
• The statistic must be in the NZ media during the period of October 1 – 7 2016 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.

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If you’d like to comment on or debate any of this week’s Stat of the Week nominations, please do so below!

• There has been some … free and frank exchange of views… this week on the question of criticising published research. The phrase “methodological terrorism” was used. Rather than linking to the combatants, I’ll give you Hilda Bastian and Jeff Leek (who have themselves had strongly-worded exchanges here and elsewhere)

• “Before analytics, businesses often had policies that every customer should be treated like they’re the best customer – because absent the data, the assumption was that every customer had that potential. But in the data age, there is no more benefit of the doubt.” Cathy Carleton. Some people (mostly economists) will probably feel that this is all good. That’s  a defensible position, but poor service for the poor wasn’t a goal of the analytics system.

• Fact checking can help (sometimes)

• Pretty isochrone (equal travel time) maps of US cities (via Alberto Cairo)

• There are people here and in the US claiming that self-selected (‘bogus’) internet polls with no reweighting or modelling give useful information. Those people are wrong. Do not be those people.