Stevens's Typology
In the metadata, the following entries are allowed for the variable attribute SCALE_LEVEL:
SCALE_LEVELSAn object of class list of length 5.
nominal for categorical variables
ordinal for ordinal variables (i.e., comparison of values is possible)
interval for interval scales, i.e., distances are meaningful
ratio for ratio scales, i.e., ratios are meaningful
na for variables, that contain e.g. unstructured texts, json,
xml, ... to distinguish them from variables, that still need to
have the SCALE_LEVEL estimated by
prep_scalelevel_from_data_and_metadata()
sex, eye color -- nominal
income group, education level -- ordinal
temperature in degree Celsius -- interval
body weight, temperature in Kelvin -- ratio