Shahul

You got it.

Always keep in mind float as computed by old CPM theory is not the same as float computed by true PDM theory. That some incapable software that never got it right are an in between that can provide you with sloppy resource leveling but cannot provide you with true float in accordance to the resource leveling functionality.

So we have:

Old CPM Software: dinosaurs, almost extinct that in its origin only provided FS relationships, no lag, no resource leveling.

The in-betweens: deficient software that can provide substandard resource leveling, no resource constrained float and therefore shown float loss its meaning after resource leveling.

True PDM Software: updated software that can provide you with better resource leveling algorithms, preferably a few as no single algorithm is "best". That can provide you with resource critical float and do not hide you the fact that these values are dependent upon the particular algorithm.

Best regards,
Rafael

Using various calendars create some confusion, if you can avoid it great. I usually avoid it because of the way we schedule our jobs but we need all the functionality relative to resource loading, we need crews, we need material resources, we need skills, multi-resources, resource production, shifts and so on.

Some CPM users do have a need to schedule for named resources each with a particular calendar of their own. One example would be IT jobs. No way they can model real needs without using many calendars, they have to deal with the difficulties this mean with regard to float.

Sorry but good software should provide you with all this, should provide you with some confusion. Who told you life is simple?

Activity float is the amount of time for which an activity may be postponed from its early start without delaying the project finish date. Float is a mathematical value, and can change as the project progresses and changes are made to the project plan. Activities with floats less than or equal to the specified value, often zero, define project critical path and are called critical activities. Execution delay of such activities leads to increase of project duration unless preventives are undertaken (usually assignment of additional resources, increase of work load and so on).

Resource float is time interval for which work of this resource may be postponed without delaying project finish date.

Each of the above are computed on their own calendar.

Activity float should always be measured against the activity calendar even if your resource(s) calendar(s) do(es) not match up exactly with the particular activity calendar. What if your particular activity calendar is based on weekend as workdays and your main calendar is based on a monday to friday work week?

Multiple calendars create confusion in the identification of critical path but this is another issue. That your resources will not be available always as per activity calendar is another one of the many factors that add to the confusion on float. Your software must provide you with resource float as well.

Lag calendars are also to be followed; as these might differ from predecessor/successor activities in the case your software allows you to define it yourself.

Finally if you perform resource leveling and your software is not capable of providing resource constrained float the measure of float will mean nothing, will be a lie.

Better get software capable of showing resource critical float, for example:

Schedule a 2 activities network, not tied one to each other by precedence logic but sharing a limited resource.

Activity A – 5 days
Activity B - 5 days
Both activities sharing Resource A.

Most software under resource leveling will schedule A and B at different time, the first one depending on your priorities selection. This is ok up to this point, but the problem is most will show float of 5 days on the first activity while a delay in this activity will delay the completion of your schedule.

Imagine that you as a Project Manager look at the available float as per your CPM schedule and start paying attention to those shown as critical and even start moving resources from those activities showing some float as to keep under control those showing no float. Because most software under resource constrained schedule will show the wrong float for resource critical activities you most probably will end up moving resources precisely from the wrong activities. You either will discover too late it was wrong or start doubting about the CPM float display. Well better start doubting about the CPM float display and either buy a glass ball or software capable of showing resource critical float.

Best regards,
Rafael

depends on the rules in the algorithm, and the software tool. very easy to test.