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Hi folks,
Critical path is the sequences of activities which their Total float is 0. When this sequence is defined, it is already late to take countermeasure,
I need to determine earlier the sequence of activities which would delay the project finish date.
Should I fix a threshold of the total float? How would this sequence be called? , sub critical path , near critical path ?...
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Another technique for tracking the likelihood of a non-critical activity becoming critical is the float burn index (FBI) of a given path between a burst point and a merge point:
Let's take two paths:
ABCDE has a duration of 40D and float of 10D.
QRSTUVW has a duration of 100D and float of 35D.
ABCDE has a maximum float burn rate (MFBR) 10 / 40 or .25.
QRSTUVW has a maximum float burn rate (MFBR) 35 / 100 or .35.
20D after the path ABCDE was scheduled to begin, its float is 8D. Its actual FBR (AFBR) is 2 / 20 or .10. Float burn index (FBI) is AFBR / MFBR, or .10 / .25 = .40. This path is currently comfortably on target to not become critical.
40D after the path QRSTUVW was scheduled to begin, its float is 15D. Its actual FBR (AFBR) is 20 / 40 or .50. Its float burn index (FBI) is .50 / .35 = 1.43. Any path with an FBI of greater than 1.0 is using up its float at a rate that will force it to become critical. At this rate, the path will add .43(35D) or 15D (of drag and drag cost!) to the project duration.
This is not the be-all and end-all of FBI theory -- an important variant is the maximum free float burn rate (MFFBR): when the merge point at the end of a path is scheduled to still have float, to subtract that float from the path's float. For example:
Path QRSTUVW is as above, but then merges into Activity X which has total float of 15D. Now the MFFBR of QRSTUVW is (35-15) / 100 or .20. This says that the maximum rate at which the path QRSTUVW can burn its float without cutting into the float of the path's successor activities that have less float is a ratio of two days in every ten, not 3.5 days in every ten. Essentially, the MFFBR is to the MFBR as free float is to total float.
A report that tracks the FBI for all paths is a nice tool for identifying and stomping out a brushfire before it becomes a conflagration.
Fraternally in project management,
Steve the Bajan
I agree with Raymund approach, his method is not dependent on location of feeding buffers that will be wrong as soon as feeding chain moves, in addition it will account for different activity calendars.
It is clean, simple and flawless.
It works perfectly with optimistic schedule approach. You can still leave critical float equal to zero and use a color marker to highlight available float value ranges on a separate column or use similar approach to change bar colors. I still prefer to leave as critical F=0 because they are the most critical and filter for critical and near critical using some range values for float and forget about the coloring book method.
It is your choice, Raymund suggestion is a good one.
You can set to Define Critical Activities with Total Float = 30 Days.... In this case Activities with Total Float within a range 0 - 29 are now within your buffer Zone.
Stephen , Gary !
thanks a lot for your contribution
Kamel,
Stephen raises some good points in that it is not too late once TF =0 -indeed you should always have a path of activities where TF=0 no matter how early or late into the project youm are.
However it is definitely a good idea to keep an eye on activities that are at risk of becoming critical in the future. I typically use 3 methods to do this:
1) focus on the 3 to 5 (depending on the project) most critical paths through the project, rather than just the 1 critical path. -This helps you see what other activies require almost as much attention to avoid slippage as the critical path.
2) Insert tracking milestones at the end of key sequences of work throughout the programme, and track the float of these milestones at regular intervals, prducing a "float trend" or "float erosion" graph over time. -This helps you understand areas of work that are consistently being delayed and are likely to become critical if action is not taken to adress this
3) Use Monte Carlo analysis to understand which risks are most likely to cause delay to project completion, and require additional effort to mitigate.
Cheers,
G
Kamel wrote:
"Critical path is the sequences of activities which their Total float is 0. When this sequence is defined, it is already late to take countermeasure,"
Actually, Kamel, it is not too late. Nor is it too late when the project is half over and your critical path has negative float of 20 days. A good scheduler should always be looking for ways to "optimize" the project schedule by identifying the the drag and drag cost of the critical path items and compressing the duration either by:
Of course, you need to know the value/cost of time in order to compute a task's drag cost, and that is something most projects don't bother to quantify.
Try reading this article from Defense AT&L Magazine titled "The Drag Efficient":
http://www.dau.mil/pubscats/ATL%20Docs/Jan_Feb_2012/Devaux.pdf
If you have questions after reading it, I'll try to answer them.
Fraternally in project management,
Steve the Bajan
Email: PPadmin@planningplanet.com
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