How to return to an Earliest Start Schedule (ESS)

In ProTrack, activities can be shifted forwards in time in various ways. Below you have some options:

• Drag the activity to another time using your mouse
• Increase the duration of an activity and all its successors will shift forwards, unless constraints prevent this
• Change the baseline start or end in the grid

BUT... when the shifted activity is put back to the original time (earlier in time), the successor activities will not automatically be placed back to their earliest time. This is to avoid that manual changes result in unwanted schedule changes for other project activities.

Consider the following example. A project with three activities (1, 2 and 3) with FS relations between 1 and 2 and between 2 and 3. Each activity has a duration of 1 day and hence the total duration is 3 days (1 starts on April 5, 2 starts on April 6 and 3 starts on April 7).

Shifting activity 2 later in time does not change the start time of activity 1, but has an impact on activity 3 that is shifted later in time. In the screenshot below, activity 2 starts on April 8 (two days shift) and hence activity 3 can only start on April 9.

There are, however, two ways to shift the successors of activity 2 (in our example activity 3) back in time, as follows:

Option 1. Return to an ESS

Go to "Scheduling -> ESS/LSS" and you will see a value greater than zero. This means that the schedule is not on its Earliest Start Schedule. To go back to the ESS, put in a value of zero, and your schedule will go back to its ESS. The activities are back to their earliest start times, i.e. April 5, 6 and 7 for activities 1, 2 and 3.

Option 2. Activity is not on its ES, but successors must be schedule ASAP

In the case of option 1, the activity that you previously shifted forwards (activity 2) as well as all its successors (activity 3) will shift back to their original ESS. In case you only want to shift the successor activities back in time, but not the activity itself, you must use option 2, as follows.

Suppose that you shift activity 2 back in time, on day earlier to April 7 but not to its ES (April 6), as displayed in the screenshot below. In that case, activity 3 could be shifted back in time to start immediately after the finish of activity 2. However, option 1 is not a good alternative since then activity 2 will be rescheduled too. 

The best way to go ahead is to temporarely add a constraint on activity 2, as follows:

• Select activity 2 and go to "Baseline -> Add Constraint"
• Select constraint "Ready Start Time" and put the constraint date to its current start (default)
• Go to scheduling menu and put a value of zero into the ESS/LSS field (see option 1)
• Delete your constraint afterwards (i.e. add constraints and put on none)

You will see that all the successors of activity 2 are placed at the earliest possible start, but activity 2 itself has not shifted, as shown in the picture.

Download the ProTrack example file here.