Summary Logic
Certain scheduling software packages include summary activities as an option. If used, summary activities should not have logic relationships because their start and finish dates are derived from lower-level activities. Therefore, there is no need for logic relationships on a summary activity in a properly networked schedule.
Summary logic hinders vertical traceability. For example, if the start or finish dates of summary activities are not derived by the planned or actual dates of lower-level activities, then the dates of summary activities may be misrepresented in higher-level versions of the schedule.11 In addition, tracing logic through summary links does not impart to management the sequence in which lower-level activities should be carried out. Figure 16 gives an example of a linked summary activity. The summary activity “Certificate of occupancy” defines the start date of “perform final HVAC inspection” rather than the activity’s start date defining the summary start date. Moreover, because the summary logic masks actual work effort relationships, it may not be clear to management that “perform final HVAC inspection,” “perform final electrical inspection,” and “perform final plumbing inspection” all depend in part on “interior finishes complete.”
Figure 16: A Linked Summary Activity
Case study 5 provides an example of how using summary links may initially make scheduling activities easier but eventually convolutes the network logic.
GAO’s analysis of the General Fund Enterprise Business System schedule found that 50 summary activities (12 percent of remaining summary activities) had predecessor links. Program Management Office schedulers used these summary links rather than linking predecessors to their numerous lower-level activities. Because many of the lower-level activities began on the same date, this made updating the schedule simpler: an updated start date for the summary activity forced that same date on all the unlinked lower-level activities.
Despite making updating easier, the technique is not considered a best practice. First, summary activities do not represent work and are used simply as grouping elements. They should take their start and finish dates from lower-level activities; they should not dictate the start or finish of lower-level activities. Second, linking summary activities obfuscates the logic of the schedule. That is, tracing logic through summary links does not impart to management the sequence in which lower-level activities should be carried out.Vertical traceability is discussed in detail in Best Practice 5.↩︎