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FM Categorical Variable

2.4 Scatterplots and Association between Numerical Variables

Scatterplots

  • Scatterplots are used to visualise data with explanatory and response variables.
  • They consist of an x-y axis with each datapoint represented as a dot above its x-value and to the right of its y-value.
  • Scatterplots can be used to see relationships between variables. These relationships can be described in terms of form, direction and strength.

Example

Picture 1

Form

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2.3 Relationships between Numerical and Categorical Variables

Discussing Relationships between Numerical and Categorical Variables

  • Begin with context: what does the data represent?
  • Compare frequencies between the categories of the categorical dataset.
  • Compare the numerical data corresponding to each category on the basis of shape, spread, centre and presence of outliers.

Note: if you cannot remember how to choose appropriate measures for centre and spread, revise the notes for 1.6 Describing Numerical Distributions.

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2.1 Response and Explanatory Variables

Explanatory Variable

  • The explanatory variable (EV) is the variable used to explain or predict another variable (the response variable).
  • By convention, the explanatory variable is plotted along the x-axis of a graph, if it is numerical.

Response Variable

  • The response variable (RV) is the variable which is explained or predicted by the explanatory variable.
  • By convention, the response variable is plotted along the y-axis of a graph, if it is numerical.

Note: both explanatory and response variables can be either categorical or numerical variables.

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