Descriptive statistics comprises those methods used to organize and describe information which has been collected. Inferential statistics comprises those methods and techniques for making generalizations, predictions, or estimates about the population from the sample. Sample People who select themselves for a sample. Two variables are confounding when their effects on a response variable can not be distinguished from one another Bias . A design is biased if it systematically favors certain outcomes.
Every individual has the same chance of being chosen for the sample.
Several independent trials each selecting a random digit. Stratified Sample Separate the population into strata. Then from each of the strata select a simple random sample. Combine these for a full sample Stratified sampling is used by professional pollsters and government statisticians. Some groups have been left out of the survey.