New South Wales Cycle Helmet study

Discussion in 'Technical Chat' started by mrc7--urcm, Jun 24, 2011.

  1. mrc7--urcm

    mrc7--urcm Guest

    There is a new study just published online which looks at the
    introduction of the compulsory cycle helmet laws in New South Wales,
    Australia in 1991.

    It's published as http://dx.doi.org/10.1016/j.aap.2011.05.029

    with restricted access for non-subscribers to the journal Accident
    Analysis and Prevention.

    To summarise however they attempted to look at the ratio of head to arm,
    or head to leg injuries for cyclists and for pedestrians for a short
    period before and after the introduction of compulsory cycle helmet
    wearing.

    ------- Quote

    Abstract The study aimed to assess the effect of compulsory cycle helmet
    legislation on cyclist head injuries given the ongoing debate in
    Australia as to the efficacy of this measure at a population level. We
    used hospital admissions data from New South Wales, Australia, from a 36
    month period centred at the time legislation came into effect. Negative
    binomial regression of hospital admission counts of head and limb
    injuries to cyclists were performed to identify differential changes in
    head and limb injury rates at the time of legislation. Interaction terms
    were included to allow different trends between injury types and pre-
    and post-law time periods. To avoid the issue of lack of cyclist
    exposure data, we assumed equal exposures between head and limb injuries
    which allowed an arbitrary proxy exposure to be used in the model. As a
    comparison, analyses were also performed for pedestrian data to identify
    which of the observed effects were specific to cyclists. In general, the
    models identified a decreasing trend in injury rates prior to
    legislation, an increasing trend thereafter and a drop in rates at the
    time legislation was enacted, all of which were thought to represent
    background effects in transport safety. Head injury rates decreased
    significantly more than limb injury rates at the time of legislation
    among cyclists but not among pedestrians. This additional benefit was
    attributed to compulsory helmet legislation. Despite numerous data
    limitations, we identified evidence of a positive effect of compulsory
    cycle helmet legislation on cyclist head injuries at a population level
    such that repealing the law cannot be justified.

    Highlights Hospital admission data used to assess the impact of
    compulsory helmet legislation. Modelling the ratio of head to limb
    injuries avoids the need for cyclist exposure data. Cyclist head
    injuries decreased more than limb injuries at time of legislation.
    Differential decrease persisted after adjusting for other factors. Our
    results make it untenable to rescind compulsory helmet laws.

    ---- End Quote

    However my own view when looking at the results in the paper are that
    the conclusions should not be as strong as the authors seem to
    want to claim.

    Firstly their data shows a downward trend in head injuries for both
    cyclists and pedestrians (admittedly a greater drop for cyclists) over
    the period studied and with similar discontinuities around the date of
    introduction of the helmet legislation. Obviously this isn't because
    pedestrians suddenly started wearing helmets so it is evidence for the
    other road safety effects that have been eluded to in other studies.

    Secondly their data also shows that the rate of change for head injuries
    measurd as a ratio to limb injuries before and after cycle helmet
    introduction is different for legs and for arms. This is true for
    pedestrians and for cyclists. This suggests to me that their is evidence
    for other effects in the data (or more likely the data is too sparse
    and the observed effects are random sampling effects for small sample
    sizes) since I cannot see why wearing a helmet should offer different
    protection from arm versus leg injuries when having an accident on a
    bicycle. Also why should the wearing of cycle helmets by cyclists alter
    the before and after ratio of head to limb injuries for pedestrians?


    Mike
     
    mrc7--urcm, Jun 24, 2011
    #1
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  2. mrc7--urcm

    nmm1 Guest

    There is also the spurious positive effect. If one does 100 solid
    statistical analyses of an effect, on average one will be significant
    at the 1% level. I remember this analysis being performed before,
    and it did NOT show up as significant. So how many times have such
    analyses been performed, not been significant, and hence not been
    published?

    Normally, one isn't overly suspicious, but this case is nearly as
    bad as the one over cannabis, where masses of effort over a long
    period was put into proving the harm that 'all informed people'
    knew had to be present. Or, in this case, the good.

    Let's see a proper repetition or cross-check.


    Regards,
    Nick Maclaren.
     
    nmm1, Jun 25, 2011
    #2
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  3. mrc7--urcm

    Danny Colyer Guest

    Nicely demonstrated by:
    http://xkcd.com/882/
     
    Danny Colyer, Jun 25, 2011
    #3
  4. mrc7--urcm

    Frank Studt Guest

    Am 25.06.2011 18:55, schrieb :

    There is a related effect at work called "publication bias". A new
    meta-analysis by Rune Elvik, an Norwegian expert on road safety shows
    pretty interesting results.

    http://www.cycle-helmets.com/elvik.pdf

    A few quotes:


    "When the risk of injury to
    head, face or neck is viewed as a whole, bicycle helmets do provide
    a small protective effect. This effect is evident only in older studies.
    New studies, summarised by a random-effects model of analysis,
    indicate no net protective effect".

    "Estimates
    based on recently published studies show much smaller effects of
    bicycle helmets on head injury and facial injury than the original
    study. In fact, in the random-effects model, there is a statistically
    non-signiï¬cant tendency for the wearing of bicycle helmets to be
    associatedwith an increase of the risk of injury. As far as neck injury
    is concerned, the tendency found in the original study for the risk
    of injury to increase when a helmet is worn is conï¬rmed when a
    new estimate is added."

    "The head, the face and the neck can be viewed as three distinct
    regions of the body. Hence, it makes sense to develop summary
    estimates of effect of bicycle helmets for the head, the face and the
    neck. These estimates are shownat the bottomof Table 2. Ingeneral,
    the estimates suggest a modest overall effect of bicycle helmets.
    In the random-effects analysis, based on the new estimates only,
    the effect vanishes entirely."

    "For all studies, based on a random-
    effects model adjusted for publication bias, the best estimate is a
    15% reduction of the risk of injury to the head, the face or the neck
    if a bicycle helmet is worn. This summary estimate is statistically
    signiï¬cant at the 5% level."
     
    Frank Studt, Jun 25, 2011
    #4
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