**Blindness**

Blinding is a design which does not allow the investigator to know what treatment the patient is receiving. The purpose of blinding is to eliminate bias in reporting the outcome of the treatment since if an investigator knows what treatment the patient is taking, he/she may in some way influence a measurement or an outcome thus shifting the outcome in one direction or another consciously or unconsciously. The ideal trial should be double blind where neither the investigator nor the patient knows what treatment the patient is taking. Placebos or dummies are used in order to achieve blindness. The placebos should match the active treatment as closely as possible in terms of size, shape, color, texture, weight, taste and smell with the active formulation.

#### **Number of testing centers**

Clinical trial should be carried out in a defined centre so as to minimize variations in the popula‐ tion and in the investigators techniques. This also avoids problems of data collection, communi‐ cation and follows up. Multi-centre trials may become necessary when studying a rare disease hence scarcity of patients or when the effect being investigated is small one e.g. when one is looking out for an interaction effect between a major condition and a minor condition.

Clinical trials designed to evaluate efficacy of new drugs should always be prospective i.e. the characteristics of the population to be studied should be identified before the study begins. For example, if one randomized all patients with heart failure to treatment with either digoxin or a new drug X and then studied the outcome over six months that would be a prospective trial. In case of control study the outcome is first identified and then comparisons are made retrospectively between the characteristics of patients who did or did not have the outcome. Such a study for instance has shown that oral anticoagulants can reduce incidences of reinfarction in patients who have already had a myocardial infarction. The case control studies may be carried out some time, after the introduction of a drug therapy in order to get some idea of its place in the overall management of the disease since the results of a case control study may prompt formal prospective trials in order to confirm original findings.
