There are all sorts of factors which affect the strength of the placebo effect Amos. I'll see if I can find some literature for you - though there may be some pointers in the last website I mentioned. I may be a while tho as Im busy for the next few days.

But off the top of my head, even things like the colour of the pill can have an effect. Something like a red pill being more effective than a blue pill, and larger pills have more of an effect than smaller ones, and injections even more so.

And one mustnt forget that conventional medicines that work better than placebo also exhibit the placebo effect. For example it has been shown that pills that have more noticable side effects often have a greater effect than those without. Which in turn raises the question that the placebo used in these studies may not be a good control, as those on the real pills are more likely to suspect they are in the experimental control group real because of the side effects. This in turn can cast doubt on the validity of the results.

Completely by the by, but I remember an experiment on conditioned learning in animals in which rats were killed by sugar water! The taste of the water had previously been paired with a poisonous substance, on numerous occasions, but not enough to kill them. Then when they were given larger doses of the sugar water without the poison, the rats reacted as if they had been given the poison in a large dose, and many of them died!