British Academy: The UK's National Academy for the Humanities and Social Sciences

BA PDF Symposium 2005

26 April 2005

Abstracts

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Dr Nick Bostrom

Observation Selection Effect: Theory and Applications

How big is the smallest fish in the pond? You take your wide-meshed fishing net and catch one hundred fishes, every one of which is greater than six inches long. Does this evidence support the hypothesis that no fish in the pond is much less than six inches long? Not if your wide-meshed net can't actually catch smaller fish.

The limitations of your data collection process affect the inferences you can draw from the data. In the case of the fish-size-estimation problem, a selection effect – the net's being able to sample only the big fish – invalidates any attempt to extrapolate from the catch to the population remaining in the water. Had your net had a finer mesh, allowing it to sample randomly from all the fish, then finding a hundred fishes all greater than a foot long would have been good evidence that few if any fish remaining were much smaller. In the fish net example, a selection effect is introduced by the fact that the instrument you used to collect data sampled from only a subset of the target population. Analogously, there are selection effects that arise not from the limitations of the measuring device but from the fact that all observations require the existence of an appropriately positioned observer. These are known as observation selection effects.

The study of observation selection effects is a relatively new discipline. In my work, I have developed the first mathematically explicit theory of observation selection effects. In this presentation, I will attempt to convey a flavour of some of the mysteries that such a theory must resolve.

Dr Nick Bostrom is a philosopher at Oxford University, with wide-ranging research interests. His book, Anthropic Bias (Routledge, New York, 2002), developed the first mathematical theory of observation selection effects. In addition to philosophy of science and the foundations of probability, he also works in theoretical and applied ethics (How can Human Nature be Ethically Enhanced, (OUP, 2005, ed.), and risk analysis (Global Catastrophic Risk, OUP, 2005, ed.). He has a background in cosmology, computational neuroscience, mathematical logic, philosophy, and artificial intelligence. He has worked briefly as a consultant for the Central Intelligence Agency (Washington, DC), and for the European Commission and the European Group on Ethics (Brussels). Before coming to Oxford, he was in the Philosophy Faculty at Yale University.