Critics of homeopathic medicine argue that much of the apparent success of its remedies is due to the placebo effect (a placebo is a substance containing no medication). The remedies work because the patients believe they will work. In rebuttal, homeopaths often refer to a study involving a herd of pedigree Friesian cows. The herd was divided into two groups of 41 cows each. One group, the treatment group, was given a homeopathic remedy against mastitis, while the control group was given a placebo. The findings showed that 10 of the cows in the control group developed mastitis, while only one in the treatment group developed it.