1) To endorse the Preliminary Business Case and the progressing of Option 1A
(Henbury Spur plus Yate Turn-back), without Constable Road Station, to the
Outline Business Case (Programme Entry).
Essentially, the report uses something called a GRIP2 (or it might have been a GRIP4) analysis of information to come up with a benefit:cost ratio of the Spur v the loop.
- To produce a benefit cost ratio of take up of particular service will involve a large number of factors. Populations, catchment areas, policy decisions, growth rates in train usage, variations in such take up by location etc etc
- Each factor will have a level of accuracy (or inaccuracy) associated with it
- The accumulation of all these factors will result in a large error
- The analysis is apparently done over a 60 year period. This will exacerbate the problem significantly.
This means you are 95% confident that your answer is correct between (Your value minus the error limit) to (Your value plus the error limit). Each value in that range is equally likely.
If the error margin is 0.7, it gives you a range of 0.65 to 2.05 (which is significant because a 'BCR' of 2 is required to qualify as a scheme.
Likewise, the BCR for the spur with WBS is 2.46. If it had an error margin of 0.7, the range would be 1.76 to 3.16. You'll note that some of these values are lower than some of those above. Given that each value in either range is equally likely, then you simply cannot say that one scheme is better than the other.
The obvious question to ask then - is what error margins/confidence limits did those advocating the Henbury spur come up with?
Answer: they didn't calculate one.