Recommended Books to Learn Primavera P6

These are the very easy-to-follow and thorough books to learn Primavera P6.

2 Types of Critical Activities in Project Schedule: Longest Path & Total Float=0

In project schedule, you can have two different types of critical activities. Both of the type are made by different ways and have different purpose in project schedule management. 

Longest Path:

This critical path shows longest activity in your schedule. Or it could be a longest set of activities in your schedule. But you may ask, why the longest activity or set of activities are critical? The answer is simple. The longest activity or the longest set of activities consume most of your project time, they most probably need most of your budget and resources. Also, since these activity (ies) are the longest, these post more risk and cause more issues in the project. Moreover, you need more attention and effort on these activities.

Look at this critical activity (Longest Path) example: 

That's the reason, that longest activity(ies) are considered longest. But you might say, that is a good reason to call an activity or activities 'critical' but that isn't a significant reason perhaps to call an activity critical. Well, I would agree with you. And that is why we have another type of critical activities, which are described below:

Total Float = 0

These critical activities have no name like 'Longest Path' but these are formed when these have no margin left. In other words, these cannot be delayed further. And if they do delay, they will delay the project. Look at the Total Float=0 critical activities example below:

Recommended for you:  Difference between Free Float and Total Float

What is Out of Sequence Activity (OOS), How is It Managed?

Out-of-Sequence (OOS) activity is an activity for which its relationship has been violated, as shown in the image below:

In the example above, activity B had FS relationship with activity C. However, activity C has been updated and assigned an actual start date before Activity B.

How Out of Sequence Activity is Managed?

OOS logic should be corrected to preserve the integrity of the schedule model unless OOS for the activity was really necessary. Often, planned sequence is not correct, hence OOS logic is retained.  Out of sequence activity can be implemented in two ways:

Retained Logic


Retained Logic is recommended to comply with activity relationship to some extent.

Progress Override

Retained Logic, Progressive Override options can be set in Primavera software.

See the image at right for better understanding of progress override and retained logic options.

Schedule vs Baseline vs Actual Information in Primavera P6

In any scheduling software you enter schedule then save baseline and enter actual information. Find out the difference between these 3 types of information.


This is the planned start, finish and duration of the activity. In any scheduling software we first enter schedule, which is how we want the activities to perform.


Once the schedule has been finalized and approved, we save/create baseline. Baseline is the exact copy of your schedule. 

The reason why we create the copy of the schedule is comparison. When the activities start and you want to compare actual start, finish and actual duration with the originally planned information, we use baseline for the comparison. 

Note that we do not compare actual progress with the Schedule, although Schedule is also the original plan and that baseline was copied from that! This is due to the reason that schedule gets rescheduled every time we enter actual values. For example, an activity has 10 day duration and until today 50% (5 days) work should be done but only 25% (2.5 days) work is done because the work could not be done due to thunderstorm. This activity will complete 2.5 days (25%) later than planned. Hence, the Scheduled finish date will change. Therefore, we can not compare actual progress with current information, we need to compare with with what was originally planned, and saved, which was in the baseline.

Note that in some scheduling software, baseline was earlier called 'Target'. It becomes easier to understand baseline when we call it Target instead of baseline, since target is what we have to achieve, once the schedule has been entered and activities have started.


This is the real progress of the activities. Actuals should ideally be meeting the original plan (baseline), but this usually does not happen. When the activities start, we enter actuals such as actual start, finish, actual duration, actual man-hours and actual costs, etc. In scheduling software, we usually only enter actual work completed, in terms of duration (effort/man hours), and the software automatically calculates the actual cost, and other actual information based on actual duration/effort/man hours you have entered.

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