Cardio vascular fitness is considered a ratio variable because it uses the 6-minute walk test. The 6-minute walk test is a test that measures the distance walked over a span of 6 minutes. Therefore it could be a certain distance over 6 minutes, which is an example of a ratio variable. Ratio variable is like an interval variable but does not consider a value of zero. It is commonly used when measuring variables like height, weight and duration. Nominal is just categorizing the variables i.e. male or female. Interval is a numerical scale and order and exact difference between the values are known.

Is the research question assessing a difference between groups or a relationship between variables?