Section 3.5 of Easy CPM looks at some of the logical scheduling errors that are easy to introduce into a schedule, and that for the most part will not show up in the automated checking tools applying test such as the DCMA 14 point assessment (see more on the DCMA assessment at: https://mosaicprojects.com.au/WhitePapers/WP1088_DCMA-14-Point.pdf)
The naming convention used below is borrowed from Miklos Hajdu. In all cases the links shown in the diagram are the controlling links, in a ‘live’ schedule there are likely to be many other links as well.
In this logical configuration, the change in the overall project duration is the opposite of any change in the activity duration.
A reduction of 1-day in the duration of activity B will lengthen the project duration by one day, an increase of 1-day will reduce the project duration by one day.
Neutral Critical Open ends (dangles) have the effect of isolating the activity duration from the schedule. The project duration is unaffected by either a 1-day decrease, or a 1-day increase in the duration of activity B. There are two variants, SS and FF:
In both cases it does not matter what change is made to activity B, there is no change in the overall duration of the project. This is one of the primary reasons almost every scheduling standard requires a link from a predecessor into the start of every activity and a link from the end of the activity to a successor, however, even with other links in place, if the control is through either of the scenarios above, the result is still the same.
Finally, for this post, any change in the duration of activity B will cause the project duration to increase.
A 1-day reduction of the duration of activity B will lengthen the project duration by one day, and an increase of 1-day will also lengthen the project duration by one day. Bi-critical activities depend on having a balanced ladder where all of the links and activities are critical in the baseline schedule. Increasing the duration of B pushes the completion of C through the FF link. Reducing the duration of B pulls the SS link back to a later time and therefore delays the start of C. The same effect will occur if the ladder is unbalanced or there is some float across the whole ladder, it is just not as obvious and may not flow through to a delay depending on the float values and the extent of the change.
There are more examples of similar logical inconsistencies included in Section 3.5 of Easy CPM. Easy CPM is designed for schedulers that know how to operate the tools efficiently, and are looking to lift their skills to the next level. The book is available for preview, purchase (price $35), and immediate download, from: https://mosaicprojects.com.au/shop-easy-cpm.php