I have read that. As I understand it - an example of PPM
Take the number of trains scheduled (over the 4 week period) - say 476 on a sample line. Cancel 48 of them, and have a further 42 run 5 or more minutes late - leaving 386 trains running within 5 minutes of schedule. 386 out of 476 is 81% - an 81% PPM.
For PPM
MAA▸ - same thing but over a 52 week period. It can be equally and technically correctly calculated by averaging out the PPMs for 13 4-week periods where the level of service is unchanged for the whole period covered.
So ...
* Sundays ARE included
* A 6 minute delay has the same numeric effect as a cancellation
* No effect on the numbers from the effect on passengers once at or beyond a 5 minute delay.