Dynamic Minimum Lag Relationship - Construction Project Scheduling and Control - page 334

Similarly, the software program calculates the maximum value for RD B , but the
user can override it with a smaller number.
When using the DML relationship, the software program should use
equation 12.4 as the default; that is, AD B is given and RD B is calculated. The
software program may give the user the option to reverse the rule.
DML RELATIONSHIP IN CPM CALCULATIONS
When you are creating a schedule for a project, obviously all activities are 0% complete
before the start of the project. The DML equation 12.4 cannot be applied, since it will
involve division by zero. For this reason, the calculations (manually or by the software
program) will implicitly include both SS and FF relationships. The SS relationship
applies until the predecessor starts, so the denominator in equation 12.4 becomes
greater than zero and the DML relationship is now active. Practically speaking, the
DML equation does not differentiate itself from the SS relationship until the successor
starts. The FF relationship may be needed only to compare results with RD B , calcu-
lated by the DML equation, where the greater value governs, as will be demonstrated
in examples 12.1 and 12.2.
However, when the user assigns a DML relationship, there should be no need to
add SS and FF relationships, as these two relationships should be automatically built
into the DML relationship, that is, in the mathematical background. More clearly:
the difference between the DML relationship and the current SS + FF combination
relationship is that the latter guards against the successor activity violating the lag with
the predecessor activity when it starts and when it ends. The DML relationship does
that but, in addition, guards against the violation of the lag during the entire duration
of the successor activity.
A DML relationship also works (with the same equations above) with interrupt-
ible (noncontiguous) activities. However, the user has to be careful in defining actual
duration (AD) and at completion duration (ACD) , so they will not include unpro-
ductive time (see note 4 of “Dicussion on example 12.1”).
Rounding Numbers : Since most schedulers use the “day” as the unit of time,
software programs need to round up RD B to the nearest larger day (or unit of
time) in equation 12.4 and truncate AD B to the nearest smaller day (or unit of
time) in equation 12.5.
Note of Caution : Percent complete may be interpreted differently by different
people in different situations. 3 Software programs have numerous types of activity
percent complete. For DML calculations, percent complete is strictly defined in
equation 12.2.
3 Review the discussion on percent complete in Chapter 7.