A general record of my ongoing battle with all forms of nonsense.

Thursday 22 July 2010

There is little evidence that it doesn’t work

When Maggie Dunn and Maggy Wallace of the Complementary and Natural Healthcare Council spoke at Leicester Skeptics in the Pub earlier this week, I certainly got the impression that they were, at least to a certain extent, able to be swayed by rational argument.

I think they genuinely took something away from the Q&A session, yet there was one thing that they repeatedly said both in the Q&A and during our dinner beforehand that was (a) important and (b) not responded to. I wish to address this point here.

When faced with points made about the fact that there was no evidence for the claims made by a lot of the practices they regulate, their response was words to the effect of “but there is little evidence that it doesn’t work”.

The argument offered in opposition to this was simply that the onus of evidence is on the person making the claim. While I agree with this, it is more of a custom in argument rather than a valid point. However, there are reasons why this custom is observed that I believe are more influential than simply stating it.

There are two points that are implicitly made when someone points out that there is “no evidence that it doesn’t work either”:
  1. That in the absence of knowledge, the probability of being right or wrong is 50/50.
  2. That in the absence of knowledge, it is ethical to take a position and communicate it authoritatively.
Both of these points are incorrect.

The human body is an incredibly complex organism, and there are potentially billions of possible medical interventions, only a small handful of which are likely to work for any given ailment.

Even if we find that a given intervention is indeed useful, the probability of it being useful for any particular disease is still small. I can think of no intervention that works for most diseases.

If you were to make one reasonable and thought-through assumption about a drug’s possible effects from extensive knowledge of chemistry and biology, there is a good chance you’re going to be wrong when you apply it to the complexities of the human body.

But if you were to make an assumption based on no knowledge whatsoever, it would be highly likely that you are wrong. What’s more, the principle of Occam’s razor dictates that the chances of you being right will diminish with the number of assumptions made.

For instance, take reflexology. The first assumption is that various parts of the body are somehow connected with pathways to various parts of the foot. The second assumption is that massaging near one end of a pathway will produce an effect at the other. The third assumption is that this effect will be clinically beneficial. The forth assumption is that reflexologists have correctly mapped which positions on the foot are connected to which organs.

The likelihood of any of these single assumptions being correct in the absence of any evidence is miniscule. But for reflexology to be effective, all of these assumptions would have to be correct.

Even if we assume that the chance of each being correct is 10%, a ludicrous overestimate, then the chances of the therapy working would be a tiny 1 in 10,000.

But, for the purposes of argument, let’s imagine we live in a strange universe where the probability of any intervention being efficacious for any disease was the same as a coin toss landing heads.

Would it then be ethical to make claims of efficacy for an untested intervention?

I think not.

By making an authoritative claim that the intervention works, you are implying that you have a greater knowledge of the intervention’s efficacy than someone who is ignorant on the subject. In the mind of a person hearing your claim, the probability of efficacy will now be significantly higher than 50/50. After all, they heard it from someone presenting themselves as an expert.

If you were to ask a person who has never studied the efficacy of reflexology if it works for arthritis, the only honest answer they would be able to give would be “I’ve never studied it but in the absence of evidence it is unlikely to be effective”. For reflexologists, having never tested their treatment’s efficacy, any other reply is dishonest.


twaza (@wassabeee on twitter) said...


Congratulations in getting Maggie Dunn and Maggy Wallace to speak at your SITP meeting. It must have been fun and interesting, and I am sorry that Leicester is to far for me to attend.

When people claim that a treatment should be used because there is no evidence that it doesn't work, they are making ethical, pragmatic, and logical errors.

Ethical error: When someone advocates a new treatment, as you explained the onus is on them to show that the treatment is safe and effective. If they do not, they mislead patients and funders and prevent them from making an informed decision.

Pragmatic error: history and science have repeatedly documented (a) that most new plausible treatments eventually turn out to be unsafe, or ineffective, or less effective than initially supposed; and (b) cognitive science has repeatedly documented a large number of cognitive biases that lead us to be overoptimistic about our pet ideas. We therefore ought to actively try to detect bias in evidence and to de-bias our interpretation of evidence.

Logical error: As you explained, it is erroneous to assume an unjustifiablly high prior probability that the treatment is safe and effective. This approach, if applied consistently, can be (and has been!) used to justify any quack medicine.

AndyD said...

It's a shame someone didn't ask the simple question "what evidence against efficacy would be good enough for you?"

In reality, there's no evidence against the idea that gifting me your house might cure your cancer - but I suspect they wouldn't accept it as a legitimate system of medicine. It's that old problem of where people choose to draw their own sceptical line.

"Nigerian philathropy? Scam!"

"Magic water? It could work!"

Verification text for this comment: "PHSHI"