Posts filed under Denominator? (87)

A Campbell Live trailer just asked “Why is the disease that’s at epidemic levels about to be targeted by government cuts?”

In what is unfortunately an urban legend, Willie Sutton was supposedly asked “Why do you rob banks?”, and replied “Because that’s where the money is”.

There is no point in the NZ medical system spending too much time and effort on assessing cost-effectiveness in idiopathic pulmonary fibrosis, Fabry syndrome, or Hashimoto’s encephalitis.  Reducing the cost of diabetes treatment, on the other hand,  can free up a lot of money to spend on other illnesses.  There’s nothing surprising or shocking about targeting diabetes for cost savings, especially in a low-cost public health system like ours.

The real question is whether the savings in money come at too high a price in patient care, not why diabetes is one of the targets.

The denominator problem shows up yet again, this time in a press release from AA insurance, leading to stories in the Dominion Post, the Herald, the Aucklander, and probably others, and a Stat of the Week nomination for the Groping Towards Bethlehem blog (via Eric Crampton).

There are statistical problems in the press release, but the newspapers came up with additional bonus examples.

The press release says

Between 2009 and 2011 AA Insurance received the highest number of burglary and theft from vehicle claims from Auckland, Hamilton, Wellington, and Christchurch.

and the Dominion Post amplifies this to

But Wellingtonians were far less likely to be burgled than their Auckland counterparts, with 31 per cent of all burglaries taking place in the Auckland region, compared with just under 9 per cent in Wellington.

Auckland has three times as many burglaries as Wellington, which sounds bad until you consider that Auckland is larger than Wellington, by a factor of about, um, three.  Using population at the last census, the rate of burglaries per capita is still higher in Auckland, but by only 20%.   If we compare Auckland to the whole population of New Zealand, the burglary rate per capita is slightly lower in Auckland; and since the Wellington rate is lower, if we combine Auckland and Wellington the rate is also lower than for the rest of the country. This tends to cast doubt on the comment

AA Insurance head of operations Martin Fox said daytime robberies were more common in big cities, where most people did not head home for lunch.

This could be true  if night-time burglaries[I assume he means burglaries, not robberies] were much more common outside big cities, but we aren’t given any data to support this, and the data we do have argues against it.

So far this is mostly fluff, but the interesting bit of news is

Security alarms had proven effective for preventing burglaries, with 60 per cent of claims between 2009-11 coming from homes without alarm systems.

AA Insurance presumably know how many of their customers have security alarms, so they might have evidence for this claim. Perhaps only  30% of insured homes lack alarm systems, so the 60% of claims from such homes is notable.   We can’t tell, because they don’t explicitly give any comparisons of rates, they don’t give information that we could use to compute rates, and they sure haven’t given us any reason to trust them on the handling of denominators.

If we did have rates, there would still be a problem of causation vs correlation.  A Ministry of Justice survey in 2004 did find lower rates of burglary in houses with alarms, but they also found

The security measure most strongly associated with lowered rates of burglary was ‘telling neighbours when everyone will be away’. As only a small proportion of burglaries occurred while the occupants were away (Section 6.5.1), presumably this measure was an indicator for a more general relationship, such as a lowered risk of burglary when neighbours are known and when neighbours look out for one another.

Last week, The Australian reported on the politics of same-sex marriage in the West Island.  The story is a bit old (I picked it up from John Quiggin), but it’s such an impressive example that it’s still worth mentioning.

Roy Morgan Single Source survey data from the middle of last year shows that over a quarter of Australians aged 14 and over — 26.8 per cent — agreed with the blunt proposition “I believe homosexuality is immoral”.

That’s a fairly small minority, and although there is variation, it’s a minority everywhere in the country. But there is this:

In 80 of the 150 federal electorates, an above-average number of people support the proposition. 

So. In about half the electorates the proportion supporting the proposition is  above the national average (but still a minority), and in about half it is below the national average. That sure tells us a lot.

Thirty-three of these 80 are Labor seats. They take in a who’s who of the ALP.

Of the half that are above the national average, the proportion held by Labor is 41%, a little less than the 48% of all seats they hold.

Family First have collected some data from the Ministry of Education on ‘fees or donations’ to NZ state schools. You can read the Herald story, or just get the press release directly, without the light editing the newspaper provides.

The figure of $250 million per year is pretty impressive. NZ has about 750,000 children in school  (EducationCounts, school roll returns), so that’s maybe $330 per child, which is quite a bit for free education.

Unfortunately, Family First thought that wasn’t a big enough number and decided it would look better as a total over four years of about a billion dollars.  There’s obviously nothing special about four years here: school donations have been an issue for much longer than that, there wasn’t a big change in laws or policies four years ago, and no-one plans budgets over four year blocks, not schools, not government, not families.  It’s bad enough when reports don’t provide any context for numbers, without going out of their way to remove it. Presumably if Family First had been able to get ten years of data (and inflation-adjust it), we would see a headline total of $2.5 billion.

Interestingly, one of the paragraphs from the press release that didn’t make it into the story said that the Ministry couldn’t provide information on the proportion of parents who actually paid the donations.  This is interesting because the Herald has collected information of this sort itself in the past, as reported in this story and the Wikipedia article. As you might expect, the proportion paying was lower in the low-decile schools, but even in the high-decile schools there were quite a few who didn’t pay.

The Dominion-Post is reporting ‘hundreds of unfit teachers in class’.  They haven’t made any attempt to scale this by the number of teachers, or compare it to other professions, or basically anything that would make the number interpretable.

The number of teachers employed at State or State Integrated schools in NZ as at April 2011 was 52460. This misses out the non-integrated private schools, but they are a small fraction (4% of students).   With 664 complaints over two years, that is a rate of 1 complaint per 158 teachers per year.  About half the complaints are dismissed.

For comparison we need other professions where the public can make complaints to independent adjudicators.

• As of June 2008 there were 8230 sworn members of the police force in NZ. In the most recent single year where data are available (2010/11), there were 2052 complaints to the Independent Police Conduct Authority, that is, 1 complaint per 4 police per year.  Half the complaints were Category 5, ie, minor, too late, or otherwise not worth proceeding with.
• As of the last Census, there were 4284 people in NZ employed as reporters, editors, or sub-editors.  This probably overstates the number of journalists relevant to the Press Council, since it includes technical editors, book editors and so on.  The Press Council received 149 complaints in 2010, the last year for which they have published a report. In that year, 65 complaints went to adjudication (1 complaint per 66 journalists per year), and about half of these were upheld.

In all three professions roughly half the formal complaints that make it to the independent adjudicators are upheld and half are dismissed, but journalists are twice as likely as teachers to receive formal complaints, and police are about forty times more likely.

It’s quite likely that the headline is literally true: there probably are hundreds of unfit teachers, but that’s likely under 1% of all teachers.  It’s worth trying to weed them out, but not without considering the costs.  In any case, the amount of fainting and clutching of pearls the situations warrants is pretty limited.

When I were young, the Serious News Outlets  probably wouldn’t have admitted the existence of extra-marital affairs by non-celebrities, let alone written an article that’s basically advertising from an infidelity website press release.

In some ways the data are better-quality than most advertorials, because the website has complete data on its NZ members.  They have even gone as far as using population sizes for NZ cities to estimate their, um, market penetration, which varied across the five main cities by as much as 0.06%.  No, that doesn’t exceed the margin of error.

The Herald’s article starts off

If your partner supports National, has a PC, drinks Coke, eats meat, has a tattoo, smokes and is a Christian, be warned – they could be a cheater.

Leaving aside the gaping logical chasm in identifying website members as representative of all ‘cheaters’, what the data actually say is that more members support National, not that more National supporters are members.   As you may recall, we determined not so long ago that more New Zealanders of all descriptions support National than any other party, so that’s what you would expect for members of the website.   The proportion of National supporters in the election was 47%, among website members it’s 33%, so National supporters are substantially less likely to be members of the website than supporters of other parties. The proportion identifying as Christian among website members is very similar to the proportion in the 2006 census.   79% of website users are on PC (vs Mac).  Again that’s a lower proportion of PCs than in the population of NZ computers (the Herald said 10% were Macs in July 2010, and for Aus+NZ combined, IDC now says 15%) but one explanation is that Macs have more of the home market than the business market.  More members drinking Coke vs Pepsi is also not surprising — I couldn’t find population figures, but Coke dominates the NZ cola market.

The story doesn’t say, but we can also be pretty confident that the website members are more likely to be Pakeha than Maori, more likely to be accountants than statisticians, and more likely to have a pet cat than a pet camel.

Stuff.co.nz is reporting that twice as many Australians were arrested overseas last year as ten years ago.  A couple of minutes searching finds an Australian Social Trends report, from the Australian Bureau of Statistics, showing that, yes, trips overseas by Australians also doubled from 2000 to 2010.

Even better would be data on total time spent overseas by Aussies.  That must be available — they collect it when you re-enter the country — but I don’t want to spend more than ten minutes on this. It’s not my day job.