Topic 3: Data Basics

First Bag Second Bag Third Bag

Observations, variables, and data matrices

Data Matrix Example of data that might be collected from a first-day survey of students: Variable Student Gender Class Yr. … Study Time 1 M SO 2 60 3 F 8 4 . 42 5 Case Observation

Types of variables All variables Quantitative Categorical Continuous Observations are numerical values that represent different magnitudes Categorical Observations belong to a set of distinct categories Continuous Possible values form an interval like [0, 10] Discrete Possible values form a set of separate numbers like 1, 2, 3, … Regular Categorical Categories have no natural ordering Ordinal Categories have a natural ordering

Examples of variables gender – categorical, regular sleep - numerical, continuous bedtime - categorical, ordinal countries - numerical, discrete dread - categorical, ordinal

independent variables

Association between variables Does there appear to be an association between a possum’s head length and skull width? skull width (mm) head length (mm)

Relationships among variables Does there appear to be a relationship between hours of study per week and GPA?

Independent Variables If there is little to no evidence that two variables are associated, then they are said to be independent.