Free float Vs Total float

by on December 1, 2011 in Course Preparation, PMP®, Project Management

Free float Vs Total float

In continuation of the PMP terminology discussions here, here and here, let’s talk about Time Management knowledge area. Here are a few.

Duration vs Effort

When we are estimating the time required to complete an activity, we use the words effort, duration etc… Is duration and effort same? NO.

Effort is the actual amount of time spent to carry out an activity. This can be expressed in terms of person hours, man days or man months. It shows the total number of hours/days/months that each person spent working on each of the activities.

Duration is the amount of time taken to carry out an activity. That is the start time and end time which is nothing but the time elapsed to complete the activity. Duration is measured in hours/days etc.. This doesn’t count the number of people working on the activity.

For example: An activity takes 4 hours to work by one person.  The person who is carrying out this activity contributes one hour per day. So the total days taken will be 4 days. Here the effort is 4 hours where as the duration is 4 days.

Duration Chart

Duration Chart

Crashing vs Fast tracking

After the arrival of total duration of the project by estimation technique, the project manager presents the schedule to the PMO/Sponsor/Customer.  But the Customer says that the duration has to be reduced without comprising on the scope of the work. Now what is the left out option for Project Manager, either he/she has to add resources or has to run the activities in parallel.

Crashing is nothing but adding resources. Of course when the crashing happens, the cost increases, because you are adding extra resources.

Crashing

Crashing

Fast tracking represents the activities to be carried out in parallel instead of series. Here the disadvantage is that, it increases the risk in the area of may be quality as the activities are performed parallel.

Forward Pass vs Backward Pass

After the arrival of critical path in the given network diagram, we need to track closely the activities which are coming under the critical path. And for the activities which are not in the critical path the tracking can be the normal way. But when we say normal, need to know what is the degree of level. This can be determined by the parameter called float or slack. To find the float or the slack we have to perform the Forward pass and Backward pass.

Forward pass is moving through the network diagram from start to finish and covering all the paths.

Backward pass is moving through the network diagram in the reverse way or from Finish to start of the network diagram and covering all the paths.

When we do the forward and backward pass, will end up with values of Late Start (LS), Late Finish (LF), Early Start (ES) and Early Finish (EF).

Float is the difference of LS and ES or LF and EF, i.e. LS-ES or LF-EF.

Free float vs Total float

Free float is an amount of time that a schedule activity can be delayed without delaying the early start of any immediately following schedule activities.

Total float is the total amount of time that a schedule activity may be delayed from its early start date without delaying the project finish date, or intermediary milestone.

In the given example below the total float for activity 3 is 5 and the free float is also 5(ES of activity 5-EF of activity 3). Similarly the free float for activity 2 is 0.

Critical Path Example

Critical Path Example

Lead vs Lag

Lead is advancing an activity in terms of schedule to overcome the risk if anything that may happens to the project.

For example: You need an equipment 20 days from now and you are importing from outside. Assume that it takes 15 days from the day of order. Instead of waiting for 5 more days from now, order now itself so that you can avoid the possible delay. That is advancing the order day by 5 days.

Lag is deliberately delaying the successor activity.

For Example: There are two activities. The first one is painting and second one is framing. After painting will give some time so that the paint will dry and then only will start framing. Deliberate delay in the successor activity is called Lag.

lead vs lag

lead vs lag

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  • OL AG ’89

    Interesting that you deduce “Crashing” costs more.
    “Crashing is nothing but adding resources. Of course when the crashing happens, the cost increases, because you are adding extra resources.”
    If you have 36000 hrs planned to do 36000 hrs of work, you have to do 36000 hrs of work. If you plan to do the work in 6 months, then you will spend at a rate of 6000 hrs/mth to complete in 6 months. If the owner wants project to be done in 4.5 months, you still have 36000 hrs of work to complete, but you have to work more per month to achieve the same results. One item that you did not consider in your “crashing” example is the overhead cost will actually be reduced, because the overall project time is reduced. You still work the same number of hours, in a shorter time so the number of personnel required is greater but the duration is reduced. 36000 = total scope, at 6 months duration you work 6000 hrs/mth or 34.8 personnel at 172hr/mth. If you decrease the duration based on the owner’s requirement to 4.5 months, you still have 36000 hrs of scope to complete, but in 4.5 months. So 36000/4.5 = 8000hr/mth or 46.5 persons per month at 172hrs/mth. This still results at total resources of 36000 hr for the project. If you add in savings for overhead/equipment rental based on 1.5 months reduction you actually SAVE costs.