[SystemSafety] Difference between software reliability and astrology

[Derek M Jones]

All,

> For the current readership, perhaps you'd care to restate the arguments that you claim "debunk" the observations therein?

Let's start with the use of a Bernoulli process in the
analysis of fault experiences.

A Bernoulli process involves an event that occurs with
some fixed probability, p.
The probability of this event not occurring is q=(1-p)
https://en.wikipedia.org/wiki/Bernoulli_distribution

A fault is experienced when some combination of input to
a program is combined with one or more mistakes in the code.

If we take one particular coding mistake, there can be multiple
sets of inputs that produce the fault experience, and these
multiple sets of inputs occur with various probabilities.
The probability of experiencing a fault is the sum of the
probabilities of these various fault inducing input sets.
This distribution is known as a Poisson binomial distribution
https://en.wikipedia.org/wiki/Poisson_binomial_distribution

If there is only one set of input values that can cause
a coding mistake to produce a fault experience, we have
a Bernoulli process.

Now the urn model

The urn model, or Polya urn model to give it its full name,
involves an urn containing some number of balls of various colors.
A ball is drawn, its color noted, and that ball along with
a ball having the same color are returned to the urn.

In the urn model, drawing, say, a black ball increases
the probability of a black ball being drawn later (because
the first draw causes an extra black ball to be added to
the urn).
https://en.wikipedia.org/wiki/P%C3%B3lya_urn_model

-- 
Derek M. Jones           Evidence-based software engineering
blog:https://shape-of-code.com