Figure 9-24. - Dependent activity.

Calculating the free float consists of subtracting the earliest start (ES) date from the latest start (LS) date, or:

Free float = LS - ES

Figure 9-22 is an example of an activity-on-node precedence diagraming method (PDM) network with total and free float calculations completed.

INDEPENDENT ACTIVITY. - An independent activity is an activity that is not dependent upon another activity to start. Activity 1, diagramed in figure 9-23, is an example of an independent activity.

DEPENDENT ACTIVITY. - A dependent activity is an activity that is dependent upon one or more preceding activities being completed before it can start. The relationship in figure 9-24 states that the start of Activity 2 is dependent upon the finish of Activity 1.

Frequently, an activity cannot start until two or more activities have been completed. This appears in the diagram as a merge or junction. In figure 9-25, Activities 3 and 4 must be completed before the start of Activity 5.

Earlier we mentioned a "burst" of activities. A burst is similar to a merge. A burst exists when two or more activities cannot be started until a third activity is completed. In figure 9-24, when Activity 2 is finished, Activities 3 and 4 may start.

Advantages of Diagraming

Precedence networks are easy to draw because all the activities can be placed on small cards, laid out on a flat surface, and easily manipulated until a realistic logic is achieved. It is also easy to show the interrela- tionships and forward progress of the activities. Just

Figure 9-25. - Merge.