The following table gives the distribution of the amounts of rainfall (in inches) for July 2015 for 50 cities.

Rainfall                                             Number of Cities
0 to less than 2 .......................................6
2 to less than 4 .....................................10
4 to less than 6 .....................................20
6 to less than 8 .......................................7
8 to less than 10 .....................................4
10 to less than 12 ...................................3

Find the mean, variance, and standard deviation. Are the values of these summary measures population parameters or sample statistics?