Hi Annabel

 

Thanks for some thought-provoking questions. I will respond with some background before commenting on your blogs and answering your question.

 

My interest in the limits of common sense arises from a difficulty we often have at CKS. We have to justify all our recommendations, and always struggle to find sciencey sounding words to explain common sense recommendations. For example, we recommend that people with Raynaud’s syndrome wear cheap gloves to prevent their hands getting cold rather than waiting until their hands get cold and then using expensive electric hand warmers. We have been reluctant to say this is common sense, because (a) it doesn’t sound very professional, and (b) we know that common sense can sometimes be disastrously wrong.

 

 

I think that I may have worked out when you can safely use common sense (but am still looking for the sciencey sounding words to say this in our “Basis for recommendation” sections).

 

To explain the limits of common sense I need first to explain (superficially) how decision-making works.

 

The theorists have a simple algorithm for making the optimum decision for treatments:

 

1.       List all the treatment options — don’t forget the option to do nothing

 

      2.    For each option, list all the outcomes that would influence the decision, and work out each option’s expected value (the probability of the outcome times its value (positive for benefits; negative for harms)).

      3.    For each option, work out its expected value (the sum of the expected values for each outcome).

 

      4.    Your best bet is to go with the option with the largest expected value — every decision is a gamble.

 

 

Unfortunately, life is a little more complicated than this.

 

      First complication: We (almost) never have all the information.

 

      Second complication: Our heads don’t work like this when making a decision. They automatically, and without thinking, find exactly 1 good reason to prefer one option, and ignore the all the other reasons for and against. Our heads just don’t like to consciously work out all the different values and probabilities of all the pros and cons. Thankfully this is seldom a problem, because most of the time our heads are very good at selecting what is a good reason to do something (see Gerd Gigerenzer’s book Gut Feeling).

 

When we use common sense we use the 1 obviously good reason to make a decision, without critical appraisal of the evidence and reasoning for the decision.  This saves us the trouble of checking that what we imagined to be a good reason isn’t actually a bad reason: maybe the treatment doesn’t work, or doesn’t work as well as another option, or maybe there could be unexpected serious adverse effects.

 

We rely on common sense all the time; decision-making would be paralyzed if we had to check everything every time. What is amazing is that common sense doesn’t lead us into trouble more often.

 

Because we have lots of evidence that common sense does sometimes lead to disaster, we should be asking ourselves more often when we can and when we shouldn’t rely on common sense. (I know of no work that is being done in this area apart from Gerd Gigerenzer and his colleagues.)

 

 

If we follow the algorithm for optimising decision-making when using common sense, we should assess its expected value.  The expected value of a common sense-based treatment decision is:

 

         The value of the benefit times its probability

   less

         The costs of any adverse effects times their probabilities

 

Because we don’t have all the information needed to calculate the expected value of common sense, we have to use our common sense.

(Sorry, I can’t resist using recursion whenever there is an opportunity!!)

 

 

To come to your examples:

 

You are right and safe to say that it is common sense not to recommend e-cigarettes. We have lots of experience of nicotine, so we know its effects, good and bad. Common sense tells us that the device is likely to be about as safe as other forms of nicotine delivery — I can’t imagine it bursting into flames — and there is little reason to think it is more effective. It would be against common sense to invest in the company that makes the device, even if they have a very creative marketing department. This is because (a) they did cruel experiments on poor little mice for no good scientific reason (they probably had to do the experiment to get a marketing licence). And, more seriously, (b) they have a job to do to show that it works in practice. You can just imagine someone, desperately craving for a cigarette, reaching for their e-cigarette and finding that the bloody battery is flat. They would relapse straight away.

 

You are right and safe to suggest trying the warm version of the cold remedy. The warm version probably tastes better, is unlikely to be more risky, and the additional contribution to your carbon footprint from putting the drink in the microwave is probably less than that generated by the last search you did on Google.

 

So, to answer your last question, common sense can be both useful and dangerous — but (and this is common sense) not at the same time. After all, it is common sense that you should trust your bank to look after your money. It is common sense that you should trust the review written by the experts. It is common sense that you should trust your doctor.

 

Michael

 

 

 

 

Hi Michael
I like your idea that common sense can be dangerous.

But, is it more complicated than that?
- eg: on my blog I recently argued that clinical decisions about "electronic cigarettes" should not be based on common sense (ie agreeing with you; http://tinyurl.com/5gzv4p , you may need to login to see comments).
- but, on the other hand, I've also recently argued that common sense can be useful http://tinyurl.com/8zybr7  [from Eccles and Sanu, Rhinology 2008 http://www.rhinologyjournal.com/article/view_abstract.php?mgzn_id=148&rtcl_id=811  ]

So, I've either undermined my previous argument, or common sense can be both useful and dangerous. What do you think? Should I change my thinking?

best wishes
Annabel

------------------------------------------------------------------------------------------------

Dr Annabel Bentley | Assistant Medical Director | Group Medical

Bupa, 15-19 Bloomsbury Way, London WC1A 2BA
T: 020 7656 2069 | F: 020 7656 2708

www.linkedin.com/in/annabelbentley

 

Please consider the environment before printing this email.

-----Original Message-----
From: Evidence based health (EBH) [mailto:[log in to unmask]] On Behalf Of Michael Power
Sent: 14 January 2009 12:40
To: [log in to unmask]
Subject: Evidence that common sense can be dangerous

A little while ago, Richard Lehman said in his weekly review, and in his inimitable way: "Not very long ago, I could confidently assert that the best way to manage atrial fibrillation was to stop the atrium fibrillating. Obvious, isn’t it? And, like so many obvious ideas in medicine, wrong."

I would like to build a list of obvious ideas in medicine that have been dangerous; for example, advice to put babies to sleep on their tummies.

Suggestions (preferably with references!!) for my list would be much appreciated.

Thanks very much.

Michael Power
Clinical Knowledge Author, Guideline Developer and Informatician Clinical Knowledge Summaries www.cks.library.nhs.uk

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