Saw this on BBC News

http://www.bbc.co.uk/news/uk-england-london-12733824

So 11th May last year was the last time that whole of the tube ran for
a whole day without a single delay.

This sounds like a bad statistic, but it doesn't mean that every line
was delayed on all of the other days. Some of the incidents are
outside TfL's control, and do happen on other supposedly good systems
like Paris (See recent thread on this topic)

I think the real figure to look at is reliability (percentage of
trains in the timetable that were actually operated) and punctuality.
What is important is the trends in these figures, are they getting
better or worse?