When an individual selected for the sample does not provide information. A variable that can only take on a set number of values. Ex. The number of spots on a die or the number of fish in a school We observe the behavior or ask people questions as they are going about their day to day lives. Ex. Call people and ask them how often they eat out. Two variables where the effect of one on the dependent variable cannot be separated from the effect of the other. Ex. If you take drugs and drink alcohol and then have a car accident, the effect of the drugs vs. the alcohol cannot be separated from each other as the cause of the accident. A finding that is unlikely to be due to random chance alone We impose some treatment on people and then observe their behavior or ask questions. Ex. Give people a drug and then do chemical analysis of their blood. Where some members of the population cannot be selected for the sample. Ex. People without phones if one is collecting data over the phone. A way to assign probabilities to ranges of events for a continuous variable.