(a) Using a table of random numbers for random sampling has the following advantages:
(1) It is applicable to all situations.
(2) You do not need to list explicitly all service units.
(b) It has the following disadvantages:
(1) You must number your service units consecutively without gaps, at least within each subset of your service units. Subsets are formed by route, type of service days, etc.
(2) You may not be able to take advantage of the serial numbers you have already assigned to your service units for internal purposes.
(3) It can be difficult to use if the serial numbers have a large number of digits.