Tuesday, October 2, 2018

Is your tail heavy?

'Is your tail heavy' is the question raised at 'critical uncertainties' in a recent post.
It might be if you are a risk with some "memory" of the immediate past.

Risk with memory? What does that mean?
  • The immediate past influences the immediate future
  • The probability of the arrival of an outcome is not time-stationary: as time passes, the probabilities change
  • The distribution of the arrival time of an outcome is "heavy tailed", meaning that (usually) with more time: if it hasn't happened it probably won't happen  
In the posting (above), an example is the expected arrival of an email: Near term, it's expected. But, if doesn't get here soon, it may not get here at all

Project consequences:

  • Simple assumptions, like symmetrical bell curves, are unlikely to give a good picture of when a risky outcome may happen
  • Testing for an unlikely outcome may be easier and more economical than you might think: run a few tests; if it doesn't fail soon (infant mortality) it likely won't for a long while. 
  • Early on, consumer electronics exhibited such behavior. (If you could make it a few days, you were likely to make it a few years) 
Who knew 
Who knew heavy tails were the cheap way out of expensive testing??!

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