What does it mean when the clinical trial double blind?

When scientists are carrying out clinical trials for a new medicine they need to have a control group to compare their results. Just in case the act of taking a tablet or pill, for example, has an affect on a patient they use a placebo in the control group of patients. The placebo has no medicinal effect and the control group takes this and can therefore be compared to a test group which are actually given the medicine. A double-blind trial means that the patients do not know who has the placebo and who has the test medicine but the doctors administering the trial also do not know which patients received the placebo or the test medicine - this is the double-blind part. This way there is no bias in the results.