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Hi,
Can negative lag be allowed in the program at any point of time. please clarify.
Thanks & Regards
Karthik
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Hi Karthik
Keeping apart the Scheduling Theory, in general lag means period of time associated between the activities .Time period should be always positive for calculation purpose ,if it is negative then it is pointless. Similarly negative lag is meaningless and to be ignored out of the program
Simon,
The use of SS would not fit my need for activity 3 to follow activity 2 as it moves as the job progress. This is why many use the negative lag, this is why many insist on it, is not by omission, is because of a need, we all know about SS relationships.
By constraining the predecessor with a double link, a FS to successors to activity 3 and a SF(neg lag) you keep the activity as well as the float. Remaining duration can vary as a matter of updating that makes a change in original duration because of actual progress, can be due to changes in activity duration when skills replacement varies and the skill has different productivity as well as for many other reasons.
The SS with positive lag will not do it, if this happens hundreds of times I would have to recompute SS value of lag manually hundreds of times as activity 1 remaining duration varies, and perhaps a few will be missed.
http://www.youtube.com/watch?v=pcXF8PXHkyg
Negative lag is available not only because it is convenient, a valid reason as pointed out by Paul Harris but because although tricky it can yield true float values, including free float, if due care is used.
In my model Activity 3 requires to have 0 FF as to follow Activity 2, I do not see any further issue with lag paths for the remaining activities that are successors of Activity 3 as in both model successors of activity 3 are linked in the same way. The need for activity 3 to follow activity 2 is real, adjusting it manually is not good enough, is Barcharting, there are genuine and correct uses for negative float other than convenience with wrong float.
Regards,
Rafael
Rafael,
In your example, you could quite easily have used start to start relationships with posative lag. Arguably, the predecessor activity could be broken into discrete steps or smaller activities which would enable a better logical diagram and allow robust movement of the plan. The use of lags reduces the amount of free float in the programme. As an alternative, you could use a start to start with a start on or before constraint. Your activity which requires the lag would have a better float path this way and would model the potential (opportunity) for an earlier start.
Simon
Although Negative Lag tends to be a trap 99% of the time I would not go that far. In the following example in order to model your need to keep Activity 3 finish to happen 1 week before start of Activity 2 no matter how it moves you have several options:
1. Using ALAP no successor: The fist option is to apply ALAP constraint to Activity 2 but it prevents you to model successors to activity 2, works and keep float values correct.
2. Using ALAP with successor: this obviously does not do the trick.
3. Using Negative Lag. Seems like it is, but is not, the float computation is wrong, activity 3 cannot be delayed without delaying the project completion if the requirement for Activity 3 to finish 1 week before activity 2.
4. Using ALAP re-linked-successor with negative lag, works and keep float values correct.
5. Using Negative Lag no successor will yield wrong float values.
If Activity 1 represents many predecessors then you will have to create a milestone to allow for the required link node.
Sorry my friends but there are no short-cuts, it is not enough to position your bars as desired, if float values are wrong at some time the model might yield wrong results. I go for a warning against the use of negative float but not an absolute prohibition. That this prohibition might do much good is no excuse. If on a closed room there are 3 persons, The Schedule, Negative Lag and The Scheduler, and one kills The Schedule, are you to behead both? Is Negative Lag the assassin or is it The Scheduler?
I would not limit others just because it is difficult for me to create valid models. Remember at times the non use of these dangerous functionalities, just because they are dangerous, might be wrong.
I suggest to avoid using it at all.
It will not produce a good result of scheduling.
Negative lag tends to be a trap 99%. Sorry my friends but there are no short-cuts, it is not enough to position your bars as desired, if float values are wrong at some time the model might yield wrong results. Still I go for a warning against the use of negative float but not an absolute prohibition.
http://www.calveyconsulting.com/files/PS02_Great_Negative_Lag_Debate.pdf
http://www.planningplanet.com/forums/primavera-project-planner-p3/414072/negative-lag (pay particular attention to Paul Harris post)
Hi Karthik,
Negative lags are used in a programme between tasks often to model a predecessor commencing earlier than the successor. Consideration should be used to having a logically linked plan with left to right logic rather than using negative lag. Should your programme be subjected to Schedule Risk Analysis, it will fail the schedule checker as lags cannot have risk or uncertainty assigned. The negative lag probably represents an un necessary overlap where the duration is uncertain. Why not have a start to start relationship with positive lag? If you have modeled your logic correctly, there should never be the need for negative lags in your programme.
Regards
Simon
Email: PPadmin@planningplanet.com
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