For my first entry I would like to comment on a very recent news story which drew my attention and made me think about risk and psychology. The story I want to talk about relates to the way in which information about risk is communicated to the wide public.
You may remember that in the last week a large amount of coverage has been devoted to HIV vaccination research due to a report of one, very successful, experiment conducted in
Following a very positive feeling associated with this success of science and the prospect of possible benefits, I began to analyze the information presented to me with scrutiny. What really triggered my suspicion was the content of the dominant number of headlines which stated that (more or less) “chances of catching the virus were 30% less for those who had taken a vaccine”. Consequently I began to wonder, Am I 30% less likely to catch HIV if vaccinated? Or maybe out of 1000 people we can expect 300 less to become infected? As a condition to understand the actual effect of a vaccination we need to comprehend that 30% is a “relative risk reduction”. What it means is that the reduction of incidences relates to the difference between people who got HIV and were not vaccinated and people who also got HIV but were vaccinated. Important here is that no information about the prevalence of HIV within a population is given as the relative risk reduction has nothing to do with it.
Let’s use the following example: if 1% of the population gets HIV during their lifetime (real probability is much lower but would make computations unnecessarily complex) we can expect 10 people out of 1000 on average to be infected. Now imagine that the new vaccination has been administered to another 1000 people and a total number of 7 people were infected. The difference of 3 people between the two conditions constitutes for the risk reduction of 30%! I hope that you agree that the effect of the vaccination suddenly doesn’t seem so great. If we add to it that the difference is actually not statistically significant and that only one experiment has been conducted with little control, then this situation becomes quite grim.
Let me present here, the effect of this vaccination in a more informative way, using “absolute risk reduction”. We said that the prevalence in the population is 1% and that it is represented by 10 cases out of 1000 random people. Now, 7 people out of 1000 is equal to .7%. Therefore the prevalence has been reduced by .3% (1% minus .7%). In other words, .3% less people will be infected with HIV if vaccinated.
It is quiet obvious that by following these very simple computations we now have a much more accurate and realistic view of a scale of change that the vaccination produces.
My question is: Why would news stations provide us with a much more confusing and misleading relative risk reduction? The headlines could read .3% risk reduction as well. I am afraid that the only reasonable answer is a very upsetting one indeed. The bigger number, effect or magnitude of an event produces stronger emotional responses, consequently drawing more attention to the news. This is an example of putting two simple psychological findings into practice. Both statistical innumeracy and human’s tendency to respond with emotions rather than reason are used to increase the attractiveness of an event.
At the end I need to state that BBC news presented the explanation of the risk reduction on an example from the study itself. It doesn’t explain however, why the headlines were still using relative risk reduction. It should be obvious that majority of people will be biased by the information that is presented to them in the first place. Much less people will be willing to bother to try and understand the meaning of the presented percentages…
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