Science: a logical system that bases knowledge on direct, systematic observation. Empirical evidence: information we can verify with our senses. Scientific sociology: the study of society based on systematic observation of social behaviour. Concept: a mental construct that represents some part of the world in a simplified form. Variable: a concept whose value changes from case to case. Measurement: a procedure for determining the value of a variable in a specific case. Operationalize a variable: specifying exactly what is to be measured before assigning a value to a variable. Validity: actually measuring exactly what you intend to measure. Cause and effect: a relationship in which change in one variable causes change in another. Independent variable: the variable that causes the change. Correlation: a relationship in which two (or more) variables change together. Spurious correlation: an apparent but false relationship between two (or more) variables that is caused by some other variable.