I am not a statistician. Please bear this in mind.
The other day on the BBC's
more or less radio programme, there was a discussion about load factors for public transport - local buses in particular.
They said that the average (mean) bus contains 9.4 passengers. When I asked, they said they had obtained this number from DEFRA (the Department for Environment, Food and Rural Affairs) and helpfully supplied a web link to the report. It is called the "2009 Guidelines to DEFRA/DECC GHG Conversion Factors for Company Recording: Methodology Paper for Emissions Factors." published in October 2009.
Sure enough, there is a table on page 34 which gives this figure - Table 30: New Emissions factors for the 2009 GHG conversion factors.
Now, I'm not sure about you, but I have been on a lot of buses. I've been on buses which are full and I've been on buses which are almost empty. Buses containing >50 passengers are not an unusual phenomena in my experience.
But this figure would have us believe that
50% of all buses contain less than 9.4 passengers. If this is true, we can say a couple of things:
1. The spread of passengers/bus is not normally distributed around the mean of 9.4 passengers - otherwise buses with >50 passengers would be an unlikely phenomena.
2. Logic would suggest that a significant proportion of buses (say an eighth to a quarter) were running with very few or no passengers.
I don't think this is true. Indeed, I'd go as far as to say it is obvious nonsense.
The note beneath the table says that the figures were from the Transport Statistics Great Britain (TSGB) prepared by the statisticians at the Department of Transport (DoT). Please note that the 9.2 passengers/vehicle figure does not appear in this report.
So, unlike
More or Less I decided I wanted to read this report to see where this figure came from. I have attached this report as well.
Table 1.1 of this report shows the 'passenger transport by node'. If you look down the columns, you can see that it states there were 50 billion passenger km by local bus and coach in 2007.
Table 6.9 shows the total local bus vehicle km. At the bottom of the table it states that 'local bus' journeys in 2007/8 were 2664 million km and 'non-local' 1511 giving a total of 4175 million km.
So to give the number of passengers per average bus, we could calculate it as 50 billion passenger km/4.175 billion vehicle km, giving 11.97 passengers/vehicle. I cannot see a figure they could have used from this table to give anything close to the 9.2 passengers/vehicle.
I also believe this is not very accurate as it includes passengers by coach - which could skew the 50 billion figure. Coaches carry fewer passengers for longer distances, after all.
Unless anyone else can see a problem with the numbers I've used, I pronounce the DEFRA use of statistics totally flawed. Not justified by the DoT statistics, not justified by any logic or common sense. The question is - if they are so slipshod at manipulating these fairly basic numbers, why should be believe them about anything else?
[edit] - after further research, I found that these figures were based on a different, more detailed report by the DoT statisticians, see here. A critical sentance from the report reads:
DfT are currently reviewing the PSV sampling and estimation methodology. The review started this
year and has initially been focusing on the imputation methodology for passenger journeys and
vehicle kilometres on local bus services (data are imputed for survey non-response and for non-
sampled local operators).
First estimates of local bus patronage and vehicle kilometres for 2007/08 and 2008/09, based on an
improved imputation methodology, are being published for the first time in this Bulletin. However,
the imputation methodology needs to be tested and improved further.
It should also be noted that the patronage figures have not been adjusted for under-recording of
ticketless passengers as in previous years. The research on which these adjustments were based is now
some years old, and needs reviewing in the view of changes in technology and increases in
concessionary journeys in recent years. There is also evidence from some Local Authorities that
adjustments for under-recording are no longer appropriate in their area, which requires further
investigation.
Which leads me to think that the DoT are being rather more careful in making claims about their data than DEFRA are in applying and interpreting it.
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