(a) Period-based sampling plans – you sample the same number of service units for each period (e.g., monthly).
(b) Interval-based sampling plans – you sample the same number of service units one each sample day, and the interval between the sample days is constant.
(c) In determining whether using interval-based sampling plans, you should consider their advantages and disadvantages:
(1) the advantage of using a sampling plan that you have been using for years,
(2) the advantage of using regular sample days throughout a year,
(3) the disadvantage of a large number of times you need to sample during a year,
(4) the disadvantage of not being able to consider the PPMT and grouping options, and
(5) the disadvantage of a likely larger annual sample size due to rounding in determining the daily sample size.