Re: Every time ..., was Re: General formula

[David Chia <rsedc@xxxxxxxxxxxxxxxxxxxxxxx>]

> 
> Ed - this is all too hard for me ... I cannot even predict how long my
> wife will be on the phone tonight or the number of times my mobile will
> drop out when using it (hands free of course) in the car.
> 
> As to certificate usage and lifetime - theory is useful - but practice
> makes perfect. 
> 
> What is the lifetime of a Bus or Train ticket... It depends on the
> attitude and hunger of the damn machines that read them...QED
> 

You seem to have missed the point. The aim is not to predict 'exactly'
the life time of any 'particular' certificate.  The exercise is to
determine if collectively the phenomon exibits predictable behaviour
and to estimate the 'expected' or the statistical maximum likelihood
value (together with the standard error of estimation if possible) so
that cost can be evaluated and alternatives can be compared.

When I buy hardware I usually also check the mean time between failure
value beside cost and warrantee.


David Chia, RMIT University