Sampling telephone area codes
Problem: The United States currently has approximately 287 Numbering Plan Areas (NPAs) in service, corresponding to geographic regions. Each NPA is identified by a three-digit code, commonly called an area code. (More are created regularly.)16 You want to choose an SRS of 20 of these area codes for a study of available telephone numbers. Label the codes 001 to 287 and use the Simple Random Sample applet or other software to choose your sample. (If you use Table B, start at line 135 and choose only the first 5 area codes in the sample.)