activity critical by SS

The unfortunate answer to this question is it depends on the software you are using and the settings. The arithmetic involved in CPM calculations is not difficult but does need understanding – to see what your software is doing download this paper: http://www.mosaicprojects.com.au/PDF/Schedule_Calculations.pdf

Ahmet,

the short answer is because the software you are using is calculating total float as either:

1) the difference between early and late start (start float), or 

2) whichever is smaller between start float and finish float. 

You I think are expecting it to calculate based on finish float.

Depending on the software, you may be able to change this setting to your preference (for example with Primavera P6, go to schedule options and it is the last but one option on the "general" tab)

Ahmet,

Your description is still not quite clear for me, but I think this is not about drag, this is about floats. In PDM technique or it is better to say in PDM model (this is the technique where SS,SF,FF,and FS relations are allowed) we differ six different types of critical activities. These are the folowing:

• neutral critical activities (either you increase or decrease the duration of a neutral critical activity by one day the length of the critical path won't change
• normal critical activities (the critical path will increase by one day for a one day increment and will decrease byone day for a one day decrement of a normal critical activity)
• reverse critical (Jerome Wiest called it perverz critical in 1980) (one day increment will result one day decrement in the length of the critical path and vice-versa)
• bi-critical (either one day  increment or oneday decrement will result in a one day  increment of trhe critical path)
• decreasing neutral- increasing normal (one day decrement will not change the critical  path while one day increment will increase the length of the critical path)
• decreasing reverse -increasing neutral (one day decrement will increase the length of the critical path by one day , and nothing change if you increase the duration by one day)

You can find three papers on this issue: Wiest was trhe first who defined 4 types of floats in 1980, then my book from 1996 or 1997 with the definition of all the existing floats, and then Pinto at all in 2000 if I remember well.

Side note: original PDM model assumption regarding the activity durations is valid. (Activities must carry out with the same intensity without interruptions) There are some generalization of the model - as Patrick has mentioned - that allows activity splitting. I do not discuss this case it would go too far. 

Miklos

Drag is also another type of float, and it also matters. DRAG is a result of some critical path calculation, without DRAG you have a static Gantt same as without any other critical path float types. 

You got to know when and how to use these float values to better understand your schedule.

Rafael wrote: "Unfortunatelly many of today software developers forgot that CPM is about Float,.."

Actually, Rafael, most software developers THINK that CPM is about float, when it really SHOULD be about critical path drag!

CPM determines the shortest duration for the project based on activities, "sprint" durations, constraints and other delays. The project duration is always exactly equal to the longest path. Float, being off the longest path, does nothing to determine the length of the project -- rather, each activity's float is calculated based, in large part, on the length of the longest path.

Drag, on the other hand, is always on the critical path and does have implications for the length of the project. For example, if an activty's duration changes during project execution, causing a change in its float, that has no impact on the project duration. However, if an activty's duration changes during project execution, causing a change in its DRAG, that does have an impact on the project duration.

In the case of Ahmet's question and the issue of the continuous activity assumption and (potentially) the reverse critical path, the question is whether (if the extra day is on the CP) the drag caused by the continuous activity assumpation is on the on the activy or on the finish constraint caused by an FF or SF predecessor. I'd prefer that the default was non-continuous, with the drag on the activity. But with the continuous assumption, the drag would be on the constraint that is causing the delay.

Fraternally in project management,

Steve the Bajan

Under contiguous model it is enough to show the float value, defined as FLEX in Spider Project. Note that an activity can have Start Flex, Finish Flex or Both.

The problem with non-contiguous model is that the split posibilities are endless and the software will not disclose what happens with each segment, does not even tells you on how many segments and what duration each, the advocates of contiguous model consider wrong practice this behaviour of non-contiguous model. 

Unfortunatelly many of today software developers forgot that CPM is about Float, otherwise it is Gantt, they forgot there are other float types that are useful. 

Rafael

If the activity type is ‘contiguous’, the duration is set and the activity constrained by the earliest of its successors.  If you software will allow, change the type to non-contiguous (or split) and the finish will sho the float.  For more on this see: http://www.mosaicprojects.com.au/PDF/Schedule_Calculations.pdf