Half an answer at
https://orr.gov.uk/__data/assets/pdf_file/0011/39971/estimates-of-station-usage-2017-18-methodological-report.pdf1.24 Sales of tickets are assumed to correspond to a number of journeys. In the case of Single and Return tickets the relationship is very clear but for period tickets, e.g. weekly/monthly/annual seasons, there are a set of journey factors have been used within the LENNON▸ system that have remained fixed for a number of years. Whilst they were likely based on reasonable estimates of ticket use made in the past it could be argued that with lifestyle and working practice changes, e.g. greater flexible working, and ticketing arrangements that they are not as representative for today’s market.
... with lots of other questions that you have not asked also answered, and loads of special cases. We are currently between the end of the 2018/19 year at the end of March and the publication of data in early December, and if you look at all the methodology and massaging in that report, you'll see why it takes so long! At least the use of the same massaging year on year does give us something if a time series to see how things are changing, even if the absolute numbers may at times be distinctly questionable!
