Transport for London measures delays in what it calls Lost Customer Hours (LCH). These are calculated by multiplying the delays in minutes by the number of passengers. TfL records all delays over 2 minutes. It uses the financial year from April to March and splits the year in 13 equal periods for performance measurement.
Data in the London Underground Performance Almanac for the last full year shows there was an average of 139,252 Lost Customer Hours per period on the Northern line.
Comparing its recent performance with the average for 2003/06 shows a reduction in LCH of 76%, the best record on the network.
The line has one of the highest proportions of disruption caused by passengers. This accounted for 27% of the delays in 2014/15 compared to 17% across the network.
Delays caused by what TfL calls “customers and public action” include commuters taking ill, items dropped on the track and people jumping in front of trains. Documents from TfL show that more than 20 people a year kill themselves on the Underground.