Observation vs experiment: observational study observes individuals and measure variables of interest but does not attempt to influence the responses. Purpose is to describe some group/situation: experiment imposes some treatment on individual in order to observe responses. Study whether treatment causes a change in the response. When our goal is to understand cause and effect, only experiments are the only source of fully convincing data : example 9. 1) drink a little but not a lot. People who drink moderate amount of alcohol have less heart disease than people who don"t drink or drink heavily. Diet and lifestyle can affect whether or not they would be prone to heart disease or not: drinking habits (explanatory variable), but is this the only reason what causes heart disease (response variable). Diet and lifestyle (lurking variable) may also be at work. Subjects, factors, treatments factors are explanatory variables in an experiments.