Supplementary Exercise 1.1 of IPS7e
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The cases/individuals (units of measurement) of the proposed study
are the subjects participating in the study. The variables are
categorical and quantitative, as follows:

- gender (categorical/binary, 2 categories/levels)
- age (quantitative)
- race (categorical, 4 categories, not ordered => nominal)
- smoker (categorical/binary, 2 categories)
- systolic blood pressure (quantitative)
- level of calcium in blood (quantitative)

Note on terminology:
- binary means categorical with two categories (yes/no, 1/0, etc)
- levels is another word for categories, for example one could say that
a binary variable has only two levels, meaning that only two distinct
categories exist.

If we further for quantitative variables distinguish between interval 
and ratio measurements (as in the Stevens textbook), we would say that
age and serum calcium levels are ratio measurements because their scales 
have a well-defined zero so that ratios between values could make sense.
It is less clear (to me) whether the systolic blood pressure should be
considered an interval or a ratio measurement, and in any case differences
between blood pressure values would probably be more meaningful than
ratios.