this argument is used a lot. the power of the bible's message is evidenced by the effect it has on people's lives, the strong families and morals within JW ranks. now whether or not families and morals are really that strong is another matter, let's leave that out for now. let's compare this to medical testing to determine the value of some new drug.
now first the sampling must be good, broad and fairly random. the sample is split randomly into two groups with one group unknowingly taking a placebo to eliminate the psychosomatic effects. the results are taken after a period and compared.
now heres whats happening in the JW case. a sampling is taken for the A group, not randomly, but from people who WANT to improve their 'health' - tainted results already. then the 'drug' is applied, but anyone that doesnt show improved 'health' or gets 'sick' after a time is dropped from the group, and in fact, put into the B group against which we will compare our group with after. the argument will of course be made that these dropped werent taking the 'drug' correctly or at all, as evidenced by poor 'health' results - begging the question. then the good 'health' of the remaining members is held up as proof of our 'drugs' power.
now can you imagine this method used in serious medical research? what would you think if your A group after all this biased tainting showed only marginally improved health over the general populace? reminds you of faith healers 'evidence' doesnt it?
mox