The number of patients needed for each arm of the experiment was estimated assuming a difference between arms of 15 percent units for treatment failure and corresponding baseline percentage of 20%. Requiring a power of 80% and a significance level of 5% the necessary number is about 300 per arm. Inflating the size by 30% to compensate for expected loss to follow up the investigators get just under 300 patients per arm. The assumptions about differences are based on the following: a) Differences larger than 10% units in virological treatment failure are indications for review and modification of treatment strategies; b) group differences in virological treatment failure less the 20% are unlikely to lead to policy changes in Vietnam. The discussion above uses a binary variable like failure/no failure. Performing analysis on quantitative variables like CD4 T-cell readings, which are more informative, will in some cases have considerably higher power than the corresponding binary.