An enumeration which identifies the interval over which the data is sampled, compared with the rising and falling thresholds, and saved in the history. Intervals will be aligned to the real world clock. For instance, hourly samples will be taken on the hour, 15- minute samples will be taken at n:00, n:15, n:30, and n:45, etc. If a valid sample can not be taken for any reason (server down, excessive timer skew, etc.) then the sample value will be set to the invalid value. If the interval is changed, any associated history will be discarded and a new history collection commenced.