Understand the difference between Total Float and Free Float in Critical Path Method according to PMBOK Guide with the help of a Network. Observe that activity has a lead time of one day, but that relationship is between activity and Total float is the difference between the finish date of the last activity on the critical path The float on those activities is called free float. The Total Float and Free Float of an activity may not always be the same based on its predecessor and/or successor activity relationships.
The estimate considers durations and resource availability calendars. To calculate early start dates, begin with the project start date and assign that date as the start date of activities that have no predecessor activities. Follow these steps to calculate the early start dates of subsequent activities, assuming finish-start relationships: Add the lag time or subtract the lead time.
Refer to the resource calendar s that applies to the people and equipment necessary for the activity, and add the number of off-days that the activity would span on those calendars. Assign the calculated date as the early start date of the successor activity. The start date in this example is Monday, November 29, John calculates the early start date for the activities.
Total Float Versus Free Float
A partial list is provided below. Compare the figure below and the figure in the next sidebar. Observe that John is willing to work on weekends, but activity 2.
Observe that activity 2. The network path from activity 1. Fortunately, there are computer programs to assist in the process, but the project manager must understand the process well enough to recognize computer errors.
Computer software must be combined with common sense or good judgment. Float Float, sometimes called slack, is the amount of time an activity, network path, or project can be delayed from the early start without changing the completion date of the project.
Total float is the difference between the finish date of the last activity on the critical path and the project completion date. Any delay in an activity on the critical path would reduce the amount of total float available on the project. A project can also have negative float, which means the calculated completion date of the last activity is later than the targeted completion date established at the beginning of the project.
If activities that are not on the critical path have a difference between their early start date and their late start date, those activities can be delayed without affecting the project completion date. The float on those activities is called free float.Joe Lukas Explains Total and Free Float
The main issue is the algorithm written by the programmers to deal with the critical path calculations of the logic. With the scheduling programs, early start and early finish dates are less flexible than hand calculation if there is a lag or non-Finish-Start relationship such as Activities B, E, H, and F. The programming of these portions of the software has not been set up to accept all the implications of these different relationships and lags. This eliminates some potential analysis from the beginning, because MSP only allows one relationship between Table 1.
Sc hedule data and activity dates f rom Microsoft project sort by early start date. Schedule data and activity dates from primavera 6 sorts by early start date. Because of this hand calculation methods and scheduling programs must be taught together so students can experience the difference of the results from hand calculation and schedule programs. Also in the industry it is a way to double check the logic of a schedule.
Critical Path and Float – Project Management for Instructional Designers
Figure 2 shows that the focus on the activities affected by non FS or activities with Lags is all that is necessary to determine if there are differences in STF and FTF. In other words, it is not necessary to hand draw the logic of the entire schedule, just those activities affected by lags and non-finish-to-start relationships.
Activity B has a 2day SFT but it was not included in the calculation because the finish date of Activity B is critical. Activity C is forced to obey the early start date because it was critical. The relationship between G and H is a finish-to-start with no lag or lead. The free float given to Activity G does not belong to it. However, because both P6 and MSP do not know what to do with the situation in Activity H they default to what they can do.
Crashing the Schedule To achieve the least expensive duration reduction, or crashing, of the schedule is to find the cheapest and shortest option the duration of the project can be reduced. This is a fairly common occurrence on projects.
The following steps are generally accepted to reduce the duration of the project to meet the revised end date. In order to find the minimum cost schedule, trade-off analysis is necessary to find the lowest cost alternatives in each step. Generally the lowest crashing cost activities are selected and crashed. Brunnhoeffer and Celik  presented a general algorithm for crashing the schedule. The first approach is preferred because it does not crash the schedule more than necessary.
This approach can be used if the early total float and finish total float for an activity are the same. However, in the schedule in Figure 2 if Activity C were to be shortened because it is listed as critical according to both P6 and MSP, it would not have an effect on the end of the total project because only the start of Activity C is critical.
Relationships that are not FS or have Lags do not lend themselves to traditional crashing methods. Table 3 lists the activities found in Figure 2 with their data needed for crashing.
While the activities listed in Tables 1 and 2 are on the critical path only two activities are entirely critical, Activities A and D. Reducing the duration of B does not affect the total project duration because the relationship from Activity C to Activity B has a greater effect than that of Activity A to Activity B.
The total project duration will not be reduced because of its constraint from Activity I. Thus it is recommended to see if the lag from Activity I can be revised. It cannot be crashed. However it will have more total float if it is crashed. However, because Activity B has a constraint enforced by Activity C to the finish date of Activity B, the project duration will not be affected.
Reducing the duration of Activity C may or may not be beneficial to the project. Reducing its duration can affect the finish date of Activity D which happens to be critical.
Calculation of Activities Relationship Floats
Otherwise it may not affect the project duration. Another example of the effects of non FS relationships is seen in Figure 3which is a portion of the logic diagram for placing a footing. The inspection needs to be completed on Day 13, however, it can start as early as Day In other words, according to the logic, there needs to be an inspection as the rebar and formwork is being placed, there also needs to be an inspection done when the Footing concrete is placed.
This problem would best be solved by splitting the inspection activity into two separate activities to take care of both the needs spelled out by the logic.
Total Float Versus Free Float
This example does serve to demonstrate some of the issues that can occur with the use of lags and non-finish-to-start relationships to take care of some logic needs of a project. As shown in Figure 2some activities in the critical paths are critical based on either their start date or finish date. Conventional crashing guidelines cannot be applied to reduce the project duration because it assumes the whole activity is critical [2,12].
Based on the guideline, reduction in activity duration on the critical path causes automatic reduction in schedule time. However, if either only the start date or finish date is critical, crashing a critical activity does not cause an automatic duration reduction in the schedule as.