Slack Calculation Error

Question

Hello,

I'm working on a complex project (500-line items) and I think there is an error on the Total Slack calculation and the critical path. I am NOT using sub-project. It's all in one plan.

There's 5 specialty teams. 4 out of the 5 teams are trying to achieve one milestone; the other (let's call it "Oddball team") is on their own for the most part where their completion date is irrelevant to the major milestone. For the "Odd ball" team's plan, it is completely independent of these other teams plans (no cross dependencies).

Oddball team's plan is scheduled to finish end of April 2019. The major milestone everyone else is scheduled for is Oct 2019. Yet, I have a task from Oddball's team that is "critical" even though it's not connected to any of the other critical tasks! It shouldn't be critical because it's scheduled to finish way before Oct 2019.

I double checked:

• There are no deadlines set ANYWHERE in my plan
• Tasks are scheduled to start ASAP

Another issue is that one of my tasks is late. It is now turned on to the critical path (which is fine). This task has a few successors that are now showing late too. Do you know why these other successor tasks aren't turning red and on critical path too?

Any ideas?

Answer 1

How do you track progress?

I went further back, way back to a newsgroup microsoft.public.project and dug this up from 2009

When you just use % Complete, say 60%, all you are doing is telling MSP that

the tasks has say 6 days of actual duration out of 10 Days total Duration.

Since you don't tell it anything else about the task, it assumes, because it

must, that the actual start was the planned (earliest) start, and that the

task has been continuously in progress for 6 Days.

If this is not what happened, you must update with the actual facts first.

Save a Baseline

Set a Status Date

Show the Tracking Gantt View

Show the Tracking Table

Show the Tracking Toolbar

Format the Gridlines to show the Status Date as a vertical red line on the

Gantt Chart.

Type in the Actual Start Date and Actual Duration.

Do not type in % Complete. The software will calculate it for you.

Have a look at the Remaining Duration and amend it up or down if you need to

re-estimate it.

Do not leave planned duration in the past. If only 3 Days were done out of 6

planned, move the unused 3 days to the right of the status date (3rd button

on the tracking toolbar).

Do not show progress in the future. If a 10 task only started 6 Days ago, it

cannot be 80% Complete, because there cannot be 8 Days of Actual Duration

(Unless you change the Remaining Duration).

Answer 2

OK. By "FS0" I mean finish - start with zero lag. Your critical path method will never work properly unless you have a "closed" network. If you google search for critical path network you will find hundreds of pictures of closed networks, where every task is on a path from the first task to the last task. It is always useful (I would say mandatory) to have a start milestone and a finish milestone, each with zero duration, and these should be the only tasks/milestones which do not have both an FS0 predecessor and a FS0 successor. So the FS0 predecessor of your 278 should be the start milestone, and the FS0 successor should be the finish milestone. You want no blank predecessors and no blank successors.

Measuring progress by % complete is a mistake. There are many other threads (with my name attached) and discussions which explain why and how the tracking should be done. For starters, see the tracking table.

Answer 3

ProjectManager21,

Trevor and John have both provided excellent advice.  You should follow it.

The heading of your question is "Slack Calculation Error," but you've provided no data to assist in answering the question.  I.e. for specific tasks, what are the predecessors, successors, early dates, late dates, actual dates, and Slack (Start, Finish, Free, Total).

Based on the extremely limited information available, I'd suspect an issue related to out-of-sequence progress.  (Task 277 started and completed 45% even though its predecessor Task 276 has not even started.  There are likely others.)  Such out-of-sequence progress throws a wrench into the backward-pass calculations.  (It's provided the only example I've ever observed where Total Slack was NOT based on the late dates.)  Unfortunately, the more recent versions (e.g. 2016, 365) of Project are less tolerant of out-of-sequence progress, resulting in a lot of incorrect zero-slack calculations.  I'd suggest that you fix the logic such that no out of sequence progress is reported.

Good luck, tom

Answer 4

Thanks for the thoughtful reply, Trevor. Here's some answers:

• All tasks have the same calendar and the same resources (we're not really using resources though. We just named the resources)
• All tasks are auto scheduled
• Not sure what you mean by FS0 predecessor. But not all tasks have predecessors or successors (most do, but there are a few that don't). See 6.10.4 as an example. The ones that I think are incorrectly on critical path DO have predecessors and successors
• I made sure not to put any predecessors or successors at the summary task level
• I do not have "show multiple critical paths" checked.

The task is late relative to the Finish Date / Baseline. For 6.10.3 it was supposed to be completed November 5th but now it's November 21st. This should in theory, be kicking out the rest of the dates if I were to change the Finish Date to say December 1st. If I were to do that, Sample Provision (6.10.3) is still the only task that is highlighted on the critical path, even though it's (directly or indirectly) linked to 6.10.5 and 6.10.6.

Answer 5

OK, there is a lot to figure out here and some of the essential information required to figure it out has not been provided, but I will have a go anyway. A screen shot of the Gantt chart view and the schedule table would help a lot.

A task will be critical if it had zero total float (slack), and the free slack and total slack are both columns which are displayed in the schedule table. Zero total float (slack) is the default but this threshold value can be changed.

Basically, if MSP says it is critical, then it is, because it has zero total slack, and that's all there is to it. But the reason could be one or several of various causes.

No deadlines - good

All asap - good

Do all tasks have the same calendar, and all resources also have the same calendar?

Are all tasks auto-scheduled?

Does every task have at least one FS0 predecessor and at least one FS0 successor?

Does every summary not have any predecessors or successors?

In file, options, advanced, have you checked "show multiple critical paths"?

Where you say a task is "late", what do you mean by that? That is, late relative to what?

I suspect that the task has no successors and you have checked "show multiple critical paths", or perhaps there is a mixture of calendars assigned, but I can't be sure because you haven't provided enough information.

Any help?