The CPM Procedure

Example 4.12 Activity Time Constraints

Often, in addition to a project start date or a project finish date, there may be other time constraints imposed selectively on the activities in the project. The ALIGNDATE and ALIGNTYPE statements enable you to add various types of time constraints on the activities. In this example, the data set WIDGET12 displayed in Output 4.12.1 contains two variables, adate and atype, which enable you to specify these restrictions. For example, the activity 'Drawings' has an 'feq' (Finish Equals) constraint, requiring it to finish on the 15th of December. The activity 'Test Market' has a mandatory start date imposed on it.

The following statements are needed to schedule the project subject to these restrictions. The option XFERVARS in the PROC CPM statement causes CPM to transfer all variables that were used in the analysis to the Schedule data set. Output 4.12.2 shows the resulting schedule.

proc cpm data=widget12 date='1dec03'd
     xfervars interval=weekday;
   activity task;
   successor succ1 succ2 succ3;
   duration days;
   aligndate adate;
   aligntype atype;
   run;
   
title 'Activity Time Constraints';
title2 'Aligned Schedule';
proc print;
   id task;
   var adate atype e_: l_: t_float f_float;
   run;

Note that the MS and MF constraints are mandatory and override any precedence constraints; thus, both the late start and early start times for the activity 'Test Market' coincide with February 16, 2004. However, the other types of constraints are not mandatory; they are superseded by any constraints imposed by the precedence relationships. In other words, neither the early start nor the late start schedule violate precedence constraints. Thus, even though the activity 'Drawings' is required to finish on the 15th of December (by the 'feq' constraint), the early start schedule causes it to finish on the 19th of December because of its predecessor’s schedule. This type of inconsistency is indicated by the presence of negative floats for some of the activities alerting you to the fact that if some of these deadlines are to be met, these activities must start earlier than the early start schedule. Such activities are called supercritical.