The irrelevance of inference: (almost) 20 years on is it still irrelevant?

The Irrelevance of Inference was a seminal paper published by Karl Claxton in 1999. In it he outlines a stochastic decision making approach to the evaluation of health technologies. A key point that he makes is that we need only to examine the posterior mean incremental net benefit of one technology compared to another to make a decision. Other aspects of the distribution of incremental net benefits are irrelevant – hence the title.

I hated this idea. From a Bayesian perspective estimation and inference is a decision problem. Surely uncertainty matters! But, in the extra-welfarist framework that we generally conduct cost-effectiveness analysis in, it is irrefutable. To see why let’s consider a basic decision making framework.

There are three aspects to a decision problem. Firstly, there is a state of the world, \theta \in \Theta with density \pi(\theta). In this instance it is the net benefits in the population, but could be the state of the economy, or effectiveness of a medical intervention in other contexts, for example. Secondly, there is the possible actions denoted by a \in \mathcal{A}. There might be a discrete set of actions or a continuum of possibilities. Finally, there is the loss function L(a,\theta). The loss function describes the losses or costs associated with making decision a given that \theta is the state of nature. The action that should be taken is the one which minimises expected losses \rho(\theta,a)=E_\theta(L(a,\theta)). Minimising losses can be seen as analogous to maximising utility. We also observe data x=[x_1,...,x_N]' that provide information on the parameter \theta. Our state of knowledge regarding this parameter is then captured by the posterior distribution \pi(\theta|x). Our expected losses should be calculated with respect to this distribution.

Given the data and posterior distribution of incremental net benefits, we need to make a choice about a value (a Bayes estimator), that minimises expected losses. The opportunity loss from making the wrong decision is “the difference in net benefit between the best choice and the choice actually made.” So the decision falls down to deciding whether the incremental net benefits are positive or negative (and hence whether to invest), \mathcal{A}=[a^+,a^-]. The losses are linear if we make the wrong decision:

L(a^+,\theta) = 0 if \theta >0 and L(a^+,\theta) = \theta if \theta <0

L(a^-,\theta) = - \theta if \theta >0 and L(a^+,\theta) = 0 if \theta <0

So we should decide that the incremental net benefits are positive if

E_\theta(L(a^+,\theta)) - E_\theta(L(a^-,\theta)) > 0

which is equivalent to

\int_0^\infty \theta dF^{\pi(\theta|x)}(\theta) - \int_{-\infty}^0 -\theta dF^{\pi(\theta|x)}(\theta) = \int_{-\infty}^\infty \theta dF^{\pi(\theta|x)}(\theta) > 0

which is obviously equivalent to E(\theta|x)>0 – the posterior mean!

If our aim is simply the estimation of net benefits (so \mathcal{A} \subseteq \mathbb{R}), different loss functions lead to different estimators. If we have a squared loss function L(a, \theta)=|\theta-a|^2 then again we should choose the posterior mean. However, other choices of loss function lead to other estimators. The linear loss function, L(a, \theta)=|\theta-a| leads to the posterior median. And a ‘0-1’ loss function: L(a, \theta)=0 if a=\theta and L(a, \theta)=1 if a \neq \theta, gives the posterior mode, which is also the maximum likelihood estimator (MLE) if we have a uniform prior. This latter point does suggest that MLEs will not give the ‘correct’ answer if the net benefit distribution is asymmetric. The loss function is therefore important. But for the purposes of the decision between technologies I see no good reason to reject our initial loss function.

Claxton also noted that equity considerations could be incorporated through ‘adjustments to the measure of outcome’. This could be some kind of weighting scheme. However, this is where I might begin to depart from the claim of the irrelevance of inference. I prefer a social decision maker approach to evaluation in the vein of cost-benefit analysis as discussed by the brilliant Alan Williams. This approach allows for non-market outcomes that extra-welfarism might include but classical welfarism would exclude; their valuations could be arrived at by a political, democratic process or by other means. It also permits inequality aversion and other features that I find are a perhaps more accurate reflection of a political decision making approach. However, one must be aware of all the flaws and failures of this approach, which Williams so neatly describes.

In a social decision maker framework, the decision that should be made is the one that maximises a social welfare function. A utility function expresses social preferences over the distribution of utility in the population, the social welfare function aggregates utility and is usually assumed to be linear (utilitarian). If the utility function is inequality averse then the variance obviously does matter. But, in making this claim I am moving away from the arguments of Claxton’s paper and towards a discussion of the relative merits extra-welfarism and other approaches.

Perhaps the statement that inference was irrelevant was made just to capture our attention. After all the process of updating our knowledge of the net benefits of alternatives from data is inference. But Claxton’s statement refers more to the process of hypothesis testing and p-values (or Bayesian ranges of equivalents), the use of which has no place in decision making. On this point I wholeheartedly agree.

 

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