Critical Path and Negative Total Slack

[Mickah]

I'm working on a large project file (~2200 lines - unfortunately can't reduce
this) and am having trouble getting a true representation of my critical
path. I'm noticing that many of the tasks on the critical path have very high
negative total slack values and also that many of these tasks have started
already. Is there a relationship between a task starting and negative total
slack? ie, Does a task's Actual Start create an inflexible constraint date
that somehow triggers negative total slack in some or all of its
dependencies? Does the negative total slack mean I need to start that many
days earlier for the task in order not to delay the project?

I'm not sure if there are any settings or constraints we've set throughout
the project which may be causing this high level of negative total slack. We
are reviewing dates and variances weekly and are understanding where we're
behind. But the critical path analysis is a stumbling block with these
negative values and how to make decisions from it - unsure of how serious
this is and how to explain it to mgt. Any action you might suggest would be
most helpful - thank you!

 

[Hisham]

Hi:

The Negative Slack shows the amount of time that must be saved so that
successor tasks are not delayed. Negative slack indicates that there is not
enough time scheduled for the task and is usually caused by constraint dates
or task dependencies. This field is available only in the Bar Styles dialog
box and is represented as a bar on the Gantt Chart.

The total slack is calculated as the smaller value of the Late Finish field
minus the Early Finish field, and the Late Start field minus the Early Start
field.

For best uses: Add the Negative Slack field as a Gantt bar style when you
want to indicate graphically on the Gantt Chart the amount of negative slack
for tasks.

Hope this helps!

Hisham
http://epmforum.wordpress.com

 

[Rod Gill]

Hi,

If two tasks are linked then the second tasks gets an actual start date
earlier than originally scheduled, then the first task gets a negative Total
Slack indicating that it should have finished earlier. Once the first task
is 100% complete, its totals slack becomes zero (its complete so it can't be
delayed therefore it can't delay the project).

The other way a task can get negative slack is if a finish date is forced on
a task earlier than if all links were obeyed. Remove all constrained dates
on tasks except for valid date constraints (EG resource availability etc.)
These constraints should be milestones under a Constraints summary task near
the top of the schedule. Each constraint milestone should have a note
explaining it. No other tasks should have a constraint unless it is an
actual start date.

--

Rod Gill
Microsoft MVP for Project

Author of the only book on Project VBA, see:
http://www.projectvbabook.com

 

[Mickah]

That helps - Thanks!

Hi,

If two tasks are linked then the second tasks gets an actual start date
earlier than originally scheduled, then the first task gets a negative Total
Slack indicating that it should have finished earlier. Once the first task
is 100% complete, its totals slack becomes zero (its complete so it can't be
delayed therefore it can't delay the project).

The other way a task can get negative slack is if a finish date is forced on
a task earlier than if all links were obeyed. Remove all constrained dates
on tasks except for valid date constraints (EG resource availability etc.)
These constraints should be milestones under a Constraints summary task near
the top of the schedule. Each constraint milestone should have a note
explaining it. No other tasks should have a constraint unless it is an
actual start date.

--

Rod Gill
Microsoft MVP for Project

Author of the only book on Project VBA, see:
http://www.projectvbabook.com

 

[DavidC]

Trevor,

Slight proviso to the use of deadlines. they are very useful when a
sequence of activities does have a deadline. An good example is in a
contract where one party must provide an output by a specific date or end of
a specific activity. That way even though in the overall context of the
project that sequence of activities are not critical, the sequence will show
the available float in relation to the deadline. This allows for an early
assessment of the ability to meet the milestone, or to renegotiate the
milestone or ignore it. The information becomes very valuable in decision
making. I agree that deadlines can give negative slack, but it is no
different to that of the overall project in terms of understanding the impact
of a particular sequence of work on a particular date.

Regards

DavidC

 

[Steve House]

I don't have any problem with negative slack appearing, as long as you don't
let it stay there. Negative slack occurs when a task is driven to complete
after its deadline OR when links from somewhere up the line of predecessors
are attempting to drive it later in the schedule than a constraint permits
it to move. We have two tasks A (10 days) and B (1 day) linked FS. A
starts 16 June, ends 27 June. B has a FNLT constraint of 20 June. The
schedule will show the link arrow coming off of A's finish and whipping back
to the left to meet up with B, stuck there on the 20th by its constraint. B
has 6 days of negative slack. What does it mean? It's a warning that we
have created a workflow and schedule that is physically impossible to
achieve! If we let it leave our desk as-is and send it up to senior
management as it stands, we will have published a promise to our
stakeholders that we have a snowball's chance in h**l of actually being able
to deliver, not a career enhancer! I can't think of a single example where
the appearance of negative slack in the plan shouldn't set off major alarms
that the schedule is in serious need of reworking. Going ahead and trying
to work it as it stands is GUARANTEED to fail sure as God made little green
apples. That's a very valuable piece of information - it give us the chance
to fix the plan before it's too late, looking for places to rearrange the
workflow, redistribute resources or hire more, etc, while there's still a
chance of succeeding at it.

 

[DavidC]

Steve,

Agree. But there are times when the schedule shows that a particular option
selected will not work, and that message needs to get through to senior
management. Example, a project I am working on has two possible routes for
approval, one will take two plus years the other one year, after the approval
has been sought there is a contract to complete another stream of work, which
will take about 2-21/2 years to complete. Management are looking at the one
year scenario. Hence I must show the impcat of that decision then management
can make the decision to renegotiate or ignore. Not my decision but I can
tell them the stories that are there, and negative slack shows a very pointed
story. Generally I find many contractors sending me schedules which show how
they intend to carry out the work and not a strict this activity must be
complete before the next one starts or a variation of that using plus and
minus lag. This often gives negative slack and I wind up having to interpret
and amend my overall schedule. It is a lack of understanding and
appreciation of scheduling (programming) and how to construct the schedule.

Regards


DavidC

I don't have any problem with negative slack appearing, as long as you don't
let it stay there. Negative slack occurs when a task is driven to complete
after its deadline OR when links from somewhere up the line of predecessors
are attempting to drive it later in the schedule than a constraint permits
it to move. We have two tasks A (10 days) and B (1 day) linked FS. A
starts 16 June, ends 27 June. B has a FNLT constraint of 20 June. The
schedule will show the link arrow coming off of A's finish and whipping back
to the left to meet up with B, stuck there on the 20th by its constraint. B
has 6 days of negative slack. What does it mean? It's a warning that we
have created a workflow and schedule that is physically impossible to
achieve! If we let it leave our desk as-is and send it up to senior
management as it stands, we will have published a promise to our
stakeholders that we have a snowball's chance in h**l of actually being able
to deliver, not a career enhancer! I can't think of a single example where
the appearance of negative slack in the plan shouldn't set off major alarms
that the schedule is in serious need of reworking. Going ahead and trying
to work it as it stands is GUARANTEED to fail sure as God made little green
apples. That's a very valuable piece of information - it give us the chance
to fix the plan before it's too late, looking for places to rearrange the
workflow, redistribute resources or hire more, etc, while there's still a
chance of succeeding at it.
--
Steve House [Project MVP]
MS Project Trainer & Consultant
Visit http://project.mvps.org/faqs.htm for the FAQs

Trevor,

Slight proviso to the use of deadlines. they are very useful when a
sequence of activities does have a deadline. An good example is in a
contract where one party must provide an output by a specific date or end
of
a specific activity. That way even though in the overall context of the
project that sequence of activities are not critical, the sequence will
show
the available float in relation to the deadline. This allows for an early
assessment of the ability to meet the milestone, or to renegotiate the
milestone or ignore it. The information becomes very valuable in decision
making. I agree that deadlines can give negative slack, but it is no
different to that of the overall project in terms of understanding the
impact
of a particular sequence of work on a particular date.

Regards

DavidC