Calculates the number of variables in a given data set.
- is the input variables data set (two dimensional array of cells, where each column corresponds to a variable).
- is the boolean array to choose the input variables in the set. If missing, all variables in X are included.
- In case the number of the size of the mask array is smaller than the number of variables in X, the array is appended with ones to match X.
- In case the number of the size of the mask array is greater than the number of variables in X, the excess elements are dropped/ignored.
- Hamilton, J .D.; Time Series Analysis , Princeton University Press (1994), ISBN 0-691-04289-6
- Tsay, Ruey S.; Analysis of Financial Time Series John Wiley & SONS. (2005), ISBN 0-471-690740
- What to do About Missing Values in Time Series Cross-Section Data , James Honaker and Gary King, American Journal of Political Science
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