You'll notice the categorical Xs are listed as stratification variables because that's exactly what we will do to study their relationship with the primary metric ... stratify the data and compare results from each grouping.
The sampling plan guides us in the number of rows of data (I.e. data points) we collect and assemble in the data file.
Numerical variables are listed as secondary metrics which we study in a different way than categorical variables.
In most cases a correlation analysis is the primary strategy for looking at their relationship with the primary metric.
KEY POINTS ABOUT DATA COLLECTION PLANNING
The key points are these:
(A) Our data collection (DC) plan is there to help us design the elements of the data worksheet.
(B) The list of variables in the DC plan - the primary metric, the categorical Xs and the numerical Xs - determine the column headings in the data worksheet.
(C) Every time we collect a data point for the primary metric, we also collect one data point for every other variable.
(D) The sampling plan guides us in how we collect the data and how many rows we collect.
(E) Because there can be a lot of variation in how people collect the numerical variables, we need to operationally define what those variables are and how they must be collected.
(F) Categorical variables don't need the same definition as the numerical, because they are observed data that makes it easy for data collectors to be consistent in what they record.