<qa>  What is the standard deviation for within-patient difference?The calculation for hypothesis 1 is based on a comparison of fitness using VO2AT before and after NAC. As this is a within-patient comparison the investigators use the paired t-test. The standard deviation for the within-patient difference is assumed to be 3.8. This was estimated from the pilot data in the Background and assumes a correlation of 0.5 between before and after values. The clinically important difference of 1 was considered conservative and feasible since the pilot data had a difference of 2.1. The calculation for hypothesis 2B is based on a dichotomisation of patients in two groups: a those who deteriorate after NAC b those who dont. To compare mortality rates between the two groups the investigators would use a chi squared testalthough a logrank test may be appropriate if dropout is an issue. Its assumed that the two groups will be approximately equal this is discussed elsewhere and that the mortality rates for the two groups are 30 and 15 respectively. 