Posts from November 2014 (27)

The police think the speed tolerance change last year worked

Last year’s Safer Summer campaign introduced a speed tolerance of 4km/h above the speed limit for all of December and January, rather than just over the Christmas and New Year period. Police reported a 36 per cent decrease in drivers exceeding the speed limit by 1-10km/h and a 45 per cent decrease for speeding in excess of 10km/h.

Fatal crashes decreased by 22 per cent over the summer campaign. Serious injury crashes decreased by 8 per cent.

According to data from the NZTA Crash Analysis System, ‘driving too fast for the conditions’ was one of the contributing factors in about 20% of serious injury crashes and 30% of fatal crashes over the past seven years. The reductions in crashes seem more than you’d expect from those reductions in speeding.

So, I decided to look at the reduction in crashes where speed was a contributing factor, according to the Crash Analysis System data.

Here’s the trend for December and January, with the four lines showing all crashes where speed was a factor, those with any injury, those with a severe or fatal injury, and those with a fatality. The reduced-tolerance campaign was active for the last time period, December 2013 and January 2014. It looks as though the trend over years is pretty consistent.

For comparison, here’s the trend in November and February, when there wasn’t a campaign running, again showing crashes where speed was listed in the database as a contributing cause, and with the four lines giving all, injury, severe or fatal, and fatal.

There really isn’t much sign that the trend was different last summer from recent years, or that the decrease was bigger in the months that had the campaign.  The trend of fewer crashes and fewer deaths has been going on for some time. Decreases in speeding are part of it, and the police have surely played an important role. That’s the context for assessing any new campaign: unless you have some reason to think last year was especially bad and the decrease would have stopped without the zero-tolerance policy, there isn’t much sign of an impact in the data.

The zero tolerance could be a permanent part of road policing, Mr Bush said.

“We’ll assess that at the end of the campaign, but I can’t see us changing our approach on that.”

No, I can’t either.

Most of the coverage of the school decile changes has worked on the basis that decile tells you everything important about government funding levels.  I had thought this was true (not being a parent or school teacher, I hadn’t studied the matter carefully). Harkanwal Singh’s new interactive in the Herald shows that this is a major oversimplification. There are ‘steps’ within the deciles, and the range across decile 1 is larger than the difference between the top of decile 1 and the bottom of decile 2.

The piece combines maps and tables: the table are much better if you want to look for particular schools, but the maps are interesting for looking at geographical trends. For example, I wasn’t expecting funding increases for schools in the Onehunga/Royal Oak/Mt Roskill area.

I complain a lot about the publicity-based surveys of varying quality that make it into the NZ media, but there’s a lot more that gets filtered out.

A journalist (who I’m not sure if I should name) sent me an example from Mitre 10

The research surveyed more than 1,500 New Zealanders on their connection to the quarter-acre dream and asked their opinions on the size of back yards and what they were doing to make the most of them.

An overwhelming 84 per cent of respondents agreed that they liked the idea of the traditional Kiwi quarter-acre paradise – a large plot of land with a standalone house on it, with plenty of room outdoors, and almost all said they would rather live on the traditional quarter-acre section than in high-density housing with reduced outdoor living spaces.

Over half of respondents felt that their outdoor living space is smaller now than what they had growing up (53%). Fifty percent of respondents attributed this to sections of land getting smaller, while 35 per cent believe houses are getting bigger, so there’s less room on a section for an outdoor living space.

The press release is a well-crafted example, with supporting evidence from QV that house sizes are increasing and quotes from a Massey University researcher — not about the survey, but about the general topic.

The survey, on the other hand, was fairly bogus. It was online, and most of the respondents got there through the Mitre 10 Facebook page.  You’d expect (and the Mitre 10 CEO has said) that the Facebook page attracts Mitre 10 customers, not necessarily a representative sample.  The report confirms this, with 88% of respondents being born in NZ, compared to about 75% of the population as a whole.

To make matters worse, here’s the reported data for the paragraphs quoted above. “Houses are bigger” and “sections are smaller” were alternative responses to the same question. You couldn’t answer that both were true — the correct answer, and the position that the report itself is pushing.

One more finding I can’t resist quoting: “The majority of Kiwis (24%) have spent between $1,000 and $5,000 on their outdoor living spaces over the past year. “

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 November 28 2014.
• Statistics can be bad, exemplary or fascinating.
• The statistic must be in the NZ media during the period of November 22 – 28 2014 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!

Clinical trials involve experimenting on humans, and so you want them to involve the minimum number of people and use the information as efficiently as possible. Part of that is committing in advance to what you expect the benefits of a treatment to be (the ‘primary endpoint’). If you got to look at the data first, and search for a favorable difference between the treated and untreated people you’d need a lot more evidence to convince people. That’s why clinical trial registration is important.

Derek Lowe, at In the Pipeline, has an unfortunate example of from biotech company studying stem cells in heart disease. Last year, the registration information at ClinicalTrials.gov said

To determine safety and the effect of intracoronary infusion of AMR-001 on myocardial perfusion (RTSS), measured by gated SPECT MPI at baseline and six months in subjects post-STEMI

and

primary endpoint includes safety of bone marrow procurement (measured by adverse events) and AMR-001 cell infusion (including incidence of re-stenosis and stent thrombosis in addition to other adverse events) as well as efficacy measured by quantitative by gated SPECT MPI specifically looking at resting total severity score)

On November 17 this year it changed

To determine safety and efficacy of intracoronary infusion of NBS10.

and

The primary endpoint includes the occurrence of AE’s, SAE’s and Major Adverse Cardiac Events (MACE) and the assessment of myocardial perfusion measured by quantitative gated SPECT MPI specifically looking at resting total severity score.

From the company’s press release

• A statistically significant mortality benefit (p<0.05) in patients treated with NBS10 (also known as AMR-001) as compared to the placebo group; there were no deaths in the treatment group.
   
• A statistically significant dose-dependent reduction in SAEs (p<0.05).
• Observation of a dose-dependent numerical decrease in MACE. MACE occurred in 14% of control subjects, in 17% of subjects of who received less than 14 million CD34 cells, in 10% of subjects who received greater than 14 million CD34 cells, and in 7% of subjects who received greater than 20 million CD34 cells.

• No meaningful difference in perfusion, as evidenced by SPECT imaging, between the treatment and the control group from baseline to 6 months in resting total severity score (RTSS) suggesting this may not be a future suitable tool to assess NBS10, which is consistent with U.S. Food and Drug Administration (FDA) guidance that mortality and MACE are the appropriate approvable endpoints to determine efficacy of a cellular therapy for cardiac disease as opposed to imaging endpoints. 

That is, the trial failed to show a change where there were looking for one,  but found evidence for a reduction in other things — apparently after they knew the results.

Nature doesn’t care about round numbers in base 10, but people do.  From @rcweir, via Amy Hogan, this is Twitter data of the number of people followed and following (truncated at 1000 to be readable). The number of people you follow is under your control, and there are clear peaks at multiples of 100 (and perhaps at multiples of 10 below 100). The number following you isn’t under your control, and there aren’t any similar patterns.

For a medical example, here are self-reported weights from the US National Health Interview Survey

The same thing happens with measured variables that are subject to operator error: blood pressure, for example, shows fairly strong digit preference unless a lot of care is taken in the measurement.

As usual about this time of year, our Honours and M.Sc. students are giving talks on their research projects

• Modelling the natural inflows to New Zealand lakes
• Refactoring the xtable package
• Invertible reproducible documents
• Bootstrap goodness of fit tests
• Convex regression
• Using the Robust Covariance Matrix Estimator to improve the precision of principal component eigenvectors in the orthogonal multivariate test
• Orthogonalised multivariate survey-weighted linear models for medical data
• Factors affecting catch composition in NZ scampi fisheries
• Factors affecting tagging mortality in snapper
• Assessment of rapid eradication assessment
• Factors explaining the low income return for education among Asian New Zealanders
• A comparison of methods used to reconcile forecasts in hierarchical time series
• Numerical methods for drawing piecewise smooth curves
• An evaluation of the Christian Broadcasting Association’s Appeal and Donorcom campaigns
• Multi-choice and true/false assessments in introductory statistics: What can they tell us about student understanding?
• Bayesian computation for exoplanet data
• Modelling an Ophthalmology Clinic Booking List System: Assumptions and Implementation
• Reinforcement Processes on Graphs
• Influence analysis on phylogeny inference
• Statistical analysis of chemical soil composition in the Wairau Valley
• Describing the world’s nations
• Parameter estimation of the coalescent in continuous space

That doesn’t sound like a very interesting headline, but an important clinical trial whose results were released today has made definite steps towards re-convincing researchers on this point.

The trial, IMPROVE-IT, looked at adding a new drug, ezetimibe, to one of the standard statin drugs for cholesterol lowering, in people who had previously had a heart attack. Ezetimibe works by blocking cholesterol absorption in the gut, a completely different mechanism to the statins, which block cholesterol synthesis. The drug had previously shown unconvincing results in a preliminary study, made even less convincing by the behaviour of the manufacturer. There was increasing uncertainty that the cholesterol-lowering effect of the statins was really how they prevented heart disease, since no other drug appeared to be able to do the same thing.

Now, IMPROVE-IT has found a reduction in heart attacks and strokes. It’s very small — only 2 percentage points, even in this high-risk group of patients — but it looks real. Given the price of ezetimibe it probably won’t be widely used immediately, but it comes off patent in a few years and then use might spread a bit.  The results are also encouraging for dietary approaches to lowering cholesterol by reducing absorption: some cereals, and spreads with plant sterols.

Other stories: Forbes, New York Times 

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 November 21 2014.
• Statistics can be bad, exemplary or fascinating.
• The statistic must be in the NZ media during the period of November 15 – 21 2014 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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