Small amounts of total slack taking tasks of critical path

[Len]

I noticed that there was a 2-month period in my 6-month project where nothing
shows up on the critical path. Thanks to another post here, I discovered
that I have a tiny bit of total slack (0.17d) on a number of tasks, which
take them off the CP. I cannot understand what is creating this total slack
as all my tasks have work and durations that are in 0.5d increments and all
tasks are "start as early as possible". Any ideas as to why I'm getting this
small total slack, making it hard for me to see what appears to me to be the
CP?

 

[Steve House [MVP]]

It's hard to say what might be causing it. One of the first things I'd look
for would be a task that has a Start No Earlier Than constraint that is
causing it to begin a bit later than it otherwise might. If you format the
Gantt chart to display the critical path, the first critical task after the
2 month period in question is a good bet to be the one causing the weird
behavior - look to see if there's an unexpected constraint that escaped your
notice or something else that makes it a fixed date task. Maybe you
accidently showed some progress on it, thus fixing its start date a skosh
later than the links now would have it. I'd also take a look at any links
inbound to that same critical task to see if there is more than one chain
driving it's position and what is going on with them that makes them
different lengths - a lag time somewhere in one of them that makes it longer
than the other branch for example and jumps over the non-critical tasks for
two months could do it.

 

[Len]

Steve,

I fixed my problem and I thought I'd share it with you because it's a
strange one that you wouldn't normally think of (and I'll bet will never come
up again).

I have my plan set to use a "US Calendar" that I've customized. In order to
account for the fact that people aren't 100% productive, I assume 6 hours/day
of productivity and have set this in the "Tools/Change Working Time/Options"
menu by setting the working hours to be "9:00 to 12:00" and "1:00 to 4:00".
I set the Standard Calendar to be 6 hours/day also, but forgot to change the
working hours from the default "8:00 to 12:00" and "1:00 to 5:00".

Meanwhile, as I build my plan and add resources to the tasks, any new
resources are getting set by default to use the standard calendar. Tasks
that have those resources assigned to it are typically showing 0.17 days (or
1/6 of a 6-hour day) of total slack. As soon as I change the resource to use
the US calendar, the slack goes away.

The other insidious thing about this is that if you set the project to a
less than 8-hour schedule, but have some resources inadvertantly working 8
hours, it shrinks the duration of those tasks assigned to those resources by
25% making the schedule look shorter than it really is.

Regards,

Len S.

 

[Steve House [MVP]]

Big alarm is going off here!!! How did you set the standard calendar to 6
hours per day without changing the working hours in Tools/ChangeWorkingTime?
If you are referring to the Tools/Options menu, Calendar page, settings for
Hours per Day & Hours per Week plus the settings for Default Start and
Default Finish, you need to be aware that those fields do NOT affect the
working hours and times of any of the calendars, Standard or otherwise.

The Hours per Day and Hours per Week settings are conversion factors to
allow you to enter durations in units other than hours. Durations are
actually stored in Project in minutes. When you enter a task duration of "3
days," Project needs to convert that into minutes before writing it into its
internal data tables. When you look at a duration value, the duration
column in the the Gantt chart for example, Project reverses the conversion,
retrieves the minutes and converts that value back into the units you like
to see the duration dispalyed in. If the Hours per Day setting is the
default 8 hours, when I enter Task X and say its duration is 3 days, Project
knows I really mean 3*8 or 24 working hours * 60 minutes per hour and stores
the value 1440.0 as the task duration. When I look at the Gantt chart, I'll
see 3 days. Now here is an interesting observation - if that task starts
Monday and we are using the default Standard calendar, it will start Mon
0800 and finish Wed 1700, 24 working hours later. BUT, if I now change the
hours per day on the calendar options page to 6 such as I bet you've done,
it will now show the duration to be 4 days EVEN THOUGH the task still starts
Mon 0800 and ends Wed 1700, no change in the schedule of the task at all.
Why? Because we've changed the conversion factor. 24 hours is 3 8-hour
workdays but 4 6-hour workdays and all we have actually changed is what we
choose to call a day. The *calendar* hasn't changed and still shows work
hours of 0800-1200 and 1300-1700, meaning each 24 hour time period according
to the clock on the wall now actually contains 1.33 working days, what we're
calling a "day," in it. We work 8 hours on each day of the week but a "day"
is defined as a 6 hour time block, thus each day of the week regular working
shift has more than 1 day's worth of work taking place in it. For Project's
calendars to make sense, the work hours defined for the Project calendar and
the hours in the "Hours per Day" field should normally be equal but changing
one does not automatically change the other.

The Default Start and Default Finish fields on that page are equally not
what they seem. All date fields in Project are actually date/time fields.
When Project calculates a value for one of those it uses the Project
Calendar to determine where on the clock to place the time. Likewise, when
you use a pull down calendar, like when you click on the pull-down button to
pick a constraint or enter the Project Start date, it also uses to Project
Calendar at the moment to determine what the start time on the selected date
will be. But if you *manually* type a date, for instance by typing the date
a task should start, *without* typing a time, Project must supply some
value. It gets the one it uses from the Default Start field on that
calendar options page. Changing that field DOES NOT change the working hour
definitions you see when you're editing the calendar in ChangeWorkingTime.