year
| The last two digits of the year. |
calls
| The number of telephone calls made (in millions of calls). |
phones
data frame has 24 rows and 2 columns.Rousseeuw, P.J. and Leroy, A.M. (1987) Robust Regression and Outlier Detection. John Wiley.
Venables, W.N. and Ripley, B.D. (1994) Modern Applied Statistics with Splus. Springer-Verlag