By José Luis Ponz-Tienda, PhD

We wish that this example will be useful for all the Plexos´ fanatics.  

For Finish-to-Start relationships (Totally critical).

• Case 1. Predecessor (Act-2) and successor (Act-3) activities are critical.
• Case 2. The successor (Act-5) activity is critical, but not the predecessor one (Act-4).

The difference between case 1 and 2 is due to the effect of calendars in the computation of times. In both cases, the lag (Additional Delay) for the finish-to-start relationship is of 1 day, but in case 2, when Act-5 tries to start on Saturday, it is delayed until Monday, creating a float in Act-4.

Note that the finish-to-start relationships use the natural calendar to compute the times.

For Start-to-start relationships (Starting critical).

• Case 1. The predecessor activities (Act-6 and Act-7) is critical by its initial split.
• Case 2. The predecessor activity (Act-10) is critical by its finishing split.

The Act-7 is fragmentable (Continuous option unchecked) and the start-to-start relationship has 1 workday for case 1, while in case 2 has 3 workdays, so Act-11 must shift to the finishing split. 

You can note that if you increase the "Additional Delay", the successor activity is delayed, but the criticality remains unchanged.

If you change the continuity condition, the activity changes from starting critical to totally critical because you are conditioning the finishing by its start, but Plexos still shows the real nature of the criticality.

Note that the start-to-start relationships use the calendar of the predecessor activity for the workdays and production level and the "natural calendar" for the "Additional Delay" to compute the times.

For Finish-to-finish relationships (Finishing critical).

• The successor activity (Act-14) is critical by its finishing split.

When the most restrictive condition is due to a finish-to-finish relationship, the successor activity can be critical by the workdays or production level established in the relationship. As in the start-to-start relationship, if you increase the "Additional Delay", the successor activity is delayed, but the criticality remains unchanged.

As in the previous section, when the continuity condition is changed in Act-14, the activity becomes fully critical.

Note that the finish-to-finish relationships use the calendar of the successor activity for the workdays and production level and the "natural calendar" for the "Additional Delay" to compute the times.

For Start-to-finish relationships.

• Case 1. The activity (Act-19) is critical by its finishing split.
• Case 2. The activity (Act-24) is critical by its initial split.
• Case 3. The activity (Act-29) is critical by its initial and finishing split but not in the middle.

The criticality established by the start-to-finish relationship is a special case in which it goes from the starting of the predecessor to the finishing of the successor (Case 1).

Cases 2 and 3, do not directly constitute a case of criticality by the start-to-finish relationship, but by the interaction between several relationships. Is especially interesting the case 3, in which Act-29 is critical by its starting and finishing, but with internal float.

Note that for the Start-to-finish relationships, the activity start is computed using the predecessor's calendar, and the finish date is computed with successor's calendar. As in the previous examples, the natural calendar is used for the "Additional Delay" to compute the times.

Between subactivities.

When subactivities are involved, the criticism works as with single activities, considering that the subactivities are always considered as non-splitting allowed, but the continuity between the subactivities into the activity is discretional.

One of the advantages of working with subactivities is that the workflow is totally controlled because the production is subdivided into packages, also known as cycles or takts (production packages), in such way that the production can be modeled in a more realistic way, flowing along the stations and inside itself, being controlled and analyzed easily.