One of our customers recently encountered the perplexing situation whereby total float was greater than zero on some of their critical activities in Primavera P6.
How can this be I hear you ask? If an activity has a total float greater than zero, by definition it cannot be critical – right?
Under normal circumstances this would be a true assumption. However there are some situations in Primavera P6 whereby a positive total flight value can occur on a critical activity. In his article we will explore this possibility and tell you how to deal with this very confounding situation.
Take a look at the following example network. In the following figure you can see three red activities, which suggests that these activities are critical. However if you glance at the total float column to the left you will notice that one of these red bars is showing a total float value of five days.
It’s also worth mentioning that the start milestone is critical too, and that is showing a total float of two days. So that we can be sure that these red activities are truly critical in Primavera P6, we have included the critical column in the table to the left. In here you can see checks against the activities that P6 considers critical.
So under what circumstances exactly is this strange conundrum able to occur? To answer this question we need to take a look at a couple of attributes concerning this particular schedule. The first attribute is notable in the table view to the left. In here you can see that there are two calendars in use, a ‘Trades – 5 Day work week’, and a ‘Trades 5 Day with May outage’. We also made the May outage calendar the default calendar so that the nonworking week is visible in the Gantt chart.
The second attribute can be seen in the Schedule dialog’s Options.
If we look at the options in the fourth section down of the General tab, we can see that the longest path has been selected as the method for determining a critical activity. When we select the longest path option Primavera P6 determines activity criticality based upon its place in the logical path through the schedule. And this fact can allow us to be seeing apparently critical activities that are displaying a total float that is greater than zero.
This fact alone however is not the cause of our conundrum, rather it is simply exposing the problem being created by the use of two different calendars. While it’s quite legitimate to use multiple calendars in any schedule, it can present problems if we attempt to mix those calendars within the logic of the schedule. That is to say activities on one calendar should not be linked to activities on a different calendar if at all possible. Doing so can generate some very perplexing results in not only the dates, but the critical path.
This is particularly true if there are varying nonwork periods between the two calendars. In our example we have a one-week outage the week of May 7. Activity ‘B’ is displayed as being critical, but has five days of total float on it. This is because its successor activity ‘D’ is assigned to the May outage calendar, whereas activity ‘B’ is on a standard five day work week calendar. This fact has driven a five-day float onto what would be considered a critical activity.
If we were to schedule this same network using the ‘Total float less than or equal to zero hours’ method, rather than the ‘Longest path’ method, we would see a different outcome.
In this next figure we have performed that calculation and we now only see two red bars, both of which have zero total float.
So there you have a very simple example of how a project schedule can contain critical activities that are showing a total float value of greater than zero. If you find yourself ever having to deal with this conundrum, check the calendars that you are using within the schedule, and make sure that they are not linked with activities on different calendars.
This can even be problematic when you are not using the longest path method and calendars with different shift end times. This situation can cause subsequent activities on different shifts to start one or more days later than you might expect based on their relationship.