In an experimental study, sometimes one group is given the treatment meant to be studied (e.g. a new drug) while another group is given a placebo or benign treatment. In an observer blind-study, those charged with measuring, recording, and assessing changes in research participants do not know which of the participants have received treatment and which have received a placebo. Knowing which group the subject belongs to could bias the researcher's observations. Using an observer-blind approach is meant to prevent this bias from occurring.