Saturday, December 26, 2015

False Positives - why we should pay more attention to them

In the last year I have been involved in two pieces of work that have sought to find patterns in data that are good predictors of project outcomes that were of interest. In one cases as the researcher, in another case in a quality assurance role, looking over someone else's analysis.

In both situations two types of prediction rules were found: (a) some confirming stakeholders' existing understandings, (b) others contradicting that understanding and/or proposing a novel perspective. The value of further investigating the latter was evident but the value of investigating findings that seemed to confirm existing views seemed less evident to the clients in both cases. "We know that...lets move on.../show us something new" seemed to be the attitude. Albeit after some time, it occurred to me that two different next steps were needed for each of these kinds of findings:

  • Where findings are novel, it is the True Positive cases that need further investigation. These are the cases where the outcome was predicted by a rule, and confirmed as being present by the data.
  • Where findings are familiar, it is the False Positives that need further investigations. These are the cases where the rule predicted the outcome but the data indicated the outcome was not present. In my experience so far, most of the confirmatory prediction rules had at least some False Positives. These are important to investigate because if we do so this could help identify important boundaries to our confidence about where and when a given rule works.
Thinking more widely it occurred to me how much more attention we should pay to False Positives in the way that public policy supposedly works. In war time, civilian casualties are often False Positives, in the calculations about the efficacy of airstrikes for example. We hear about the number of enemy combatant killed, but much less often about the civilians killed by the same "successful" strikes. There are many areas of public policy, especially in law I suspect, where there are the equivalent of these civilian deaths, metaphorically if not literally. The "War on Drugs" and the current "War on Terrorism" are two that come to mind. Those implementing these policies are preoccupied with the numbers of True Positives they have achieved and with the False Negatives i.e the cases known but not yet detected and hit. But counting False Positives is much less so in their immediate interest, raising questions of if not by them, then by who?

Some Christmas/New Year thoughts from a dry, warm, safe and secure house in the northern hemisphere...

PS : see

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