Bias

• Selection bias, nonrandom assignment to study group, or subjects are allocated in a group without regard their individual characteristics than influenced in the results.
• The more ill subjects are in one group, the less ill subjects in the other.
• Recall bias, knowledge of presence of disorder. (see video) 
• To know to have a disease and don't want to answer questions.
• To have a children with a congenital disease and recall previous risk factors.
• Sampling bias, subjects are not representative relative to general population.
• Late-look bias, information gathered at an inappropriate time.
• Using a survey to study a fatal disease in a patient who is alive. 
• Procedure bias, subjects in control and study group does not receive the same treatment.
• Confounding bias, the effect of one risk factor distorts the effect of the other.
• Smokes distorts the cholesterol effect (alone) in acute myocardial infarcts.
• Lead-times bias, early detection of a disease, before it appeared by its natural history. 
• HIV detection before it become AIDS, treatment changes disease's natural history. 
• Pygmalion effect, researcher's belief in the efficacy of a treatment changes the outcome of the treatment.
• Hawthorne effect, group being studied changes its behavior to meet the expectations of the researcher.
• Patients answer yes to every risk factor to give a positive disease.
• Detection bias, more information is attempted to obtain from a exposed group than the control group.
• Allocation bias, subjects are not assigned to study in a non-random fashion.