A lot of times people get confused when they see their periodic histograms and curves (actual and/or remaining) behave in a strange way. For instance, if the actual histogram was at 6 units in the previous update then how does it show 5 units this week? Shouldn’t the actual be fixed? Yes, it should and it is. The only issue is the distribution of cumulative actual per unit time period is not fixed (if Store Period Performance is not applied. This is covered in a separate post). It is fairly obvious that this per time period distribution will vary as the total time varies which is the characteristic of in-progress activities over several updates. Refer the picture below which shows initially planned loading of a resource.
The time-wise distribution of the above loading looks like this:
Let us designate this schedule as a baseline with its copy as current update and status it for the first week as follows (I will call this Update 1, or U1):
To make the example as simple as possible I have used Duration as Percent Complete Type and updated the first activity with 30% progress in the first week. Notice the Budgeted Units (BU) distribution of this activity and those of the remaining two. The Baseline (BL) is assigned but it is clear that the BU histogram is derived from the current copy of the schedule. The distribution of the in-progress activity mirrors the BL distribution because it is based on the image just prior to the update process. In other words, it is depicting the budgeted distribution of the in-progress activity. The histogram of the other two activities is also depicting their budgeted distribution taking into account their shift in time. Throw in the Remaining Units (RU) as well and the image looks like this:
Actual Units (AU) are 30 over 5 days, i.e., 6 per day.
The sum of RU and AU equals BU and the distribution of RU corresponds with the remaining portion of this in-progress activity. For the other two not-yet-started activities, RU distribution is the same as BU distribution.
Now we update the same copy (U1) for the second week and call it U2. Here we see that our in-progress activity has progressed by 50%.
In this update, 10 (working) days have elapsed and AU = 50, i.e., 5 per day. RU is now 10 per day for the remainder of the duration which still holds true to the relation AU + RU = BU.
This explains the reduction in AU per unit of time from the previous update(s) which happens on account of in-progress activities. In the above, only histograms have been shown but it is obvious that depicted histogram redistribution translates to corresponding changes in the cumulative curve.