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Re: Best EBM methods papers for 2005?

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Sat, 11 Mar 2006 12:29:41 +0000

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 ```Some comments on NNT, NTN and ITI. The paper by Bogaty with a proposal for a new statistic, the number treated needlessly (NTN) or the index of therapeutic impotence (ITI), is about an idea, rather than a method. So, when I proposed it, I was pushing the limits of the inclusion criteria for best EBM methods paper. The reason that I found the paper memorable is that it exposes an asymmetry in the way that people react to statistics. Such asymmetries are well documented and complicate decision-making. They are therefore important tools in the spin doctor's toolkit. What I mean by asymmetry is the psychological asymmetry in interpreting information which can be presented as either a statement or its "converse". There is a mathematical symmetry between a statement and its "converse", similar to a mirror image. The "converse" carries exactly the same information that the original statement did, but presents it in a different way. For example, I could say: "The probability of X happening is p". Or I could say: "The probability of X not happenning is q". Because we know that p + q = 1, the two statements carry exactly the same information. Given one statement, we can work out what the other statement must say. The 2 statements have a logical symmetry. But at the same time they have a psychological asymmetry. I think that NNT puts a positive spin on the facts. It seems to appeal to the optimistic/gullable side of human nature. In contrast, NTN or ITI puts a negative spin on the same facts. It seems to appeal to the pessimistic/skeptical side of human nature. If you are trying sell the benefits of a treatment, counter fears, and boost hopes, use the NNT. If you are cost aware and risk averse, look at the NTN/ITI. And, if you really want to make the "best" decision, based on the best understanding of the best evidence, you need to look at the facts from all angles. Or do you? Unfortunately a simple 2X2 table generates a lot more statistics than the 4 original data items. A paper that presented all the statistics from a 2X2 table would have an effect similar to looking at 4 simple coloured shapes through a kaleidoscope. Amazing. Pretty. But concealing, rather than revealing the underlying meaning. The scientific community will have to decide through trial, error, and debate what constitutes the best set of summary statistics. But I like the idea of tying spin doctors in knots by coupling a positive spinning statistic with a negative spinning statistic. E.g. RRR with ARR; NNT with ITI. Michael (with no apologies for bringing rhetoric into EBM)```