Erlang's 99.9999999% (nine nines) reliability

Question

Erlang was reported to have been used in production systems for over 20 years with an uptime percentage of 99.9999999%.

I did the math as the following:

Answer 1

The 99.9999999% availability figure is an often-quoted but fundamentally misleading statistic. Mats Cronqvist, one of the AXD-301 team members, gave a presentation (video) (which I attended) at the 2010 Erlang Factory conference in San Francisco, discussing this precise availability statistic. According to him, it was claimed by British Telecom for a trial period (I believe from January to September 2002) of "5 node-years" using the AXD-301. There were 14 nodes carrying live traffic by the end of the trial.

Cronqvist specifically stated that this is not representative of the entire AXD-301 history, or Erlang in general, and that he was not happy that Joe Armstrong kept quoting this, leading to overblown expectations of Erlang's reliability. Others have written that five nines is a more realistic figure.

It should be stated that I am a fervent Erlang supporter and developer, who believes that the expert use of Erlang can indeed lead to very highly available systems, but just wants to reduce the hype. I of course assume that Cronqvist's representation of the facts is accurate, and have no reason to believe otherwise.