We tested the sensitivity of the value of the output of transport to:

• the effect of sampling variability - the total value could be 3 per cent higher or lower in 2000, due to sampling variability;
• assuming different percentages of 'dead time' - if there is no dead time,  the total value increases by 5 per cent;
• assuming that 50 per cent of single person escort journeys (returning from accompanying a child or adult) no longer take place - this reduces the total value by 1.4 per cent.