I just happened upon a study that was trying to prove whether drinking cranberry juice really helped prevent urinary tract infections.
They took a group of older people who were in the hospital anyway and gave half cranberry juice and half a similar control beverage and then measured how many UTIs they got.  The cranberry juice drinkers developed half as many infections, but their numbers weren't large enough to prove that cranberry juice had any significant protective power.  In a useful contribution, they add that they were able to prove that yes, older people like drinking cranberry juice.