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# Relationship B/W SPI and Total Float

1 reply [Last post]
Hafiz Khalid Hussain
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Joined: 13 Jun 2012
Posts: 5
Groups: None

Hi,

I explored the relationship between schedule performance index and total float and found,

CASE NO 1.

SPI < 1 then,

i) TF = 0 (It means on schedule but delay in non-critical activities)

ii) TF > 0 (It means ahead of schedule but delay in non-critical activities)

iii) TF < 0 (It means behind the schedule and delay in both critical and non critical activities)

CASE NO 2

SPI = 1 then,

i) TF = 0 (On schedule)

ii) TF > 0 (It means ahead of schedule but critical activities are on schedule)

iii) TF < 0 (It means non-critical activities on schedule but critical activities are delayed)

CASE NO 3

SPI > 1 then,

i) TF = 0 (It means on schedule)

ii) TF > 0 (It means bot critical and non critical activities ahead of schedule)

iii) TF < 0 (It means non critical activities ahead of schedule but critical activities are delayed)

The major purpose for sharing this information is to correct myself if i am wrong. It will be appriciated if someone give his time to read and varify. Thx

## Replies

Stephen Devaux
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Joined: 23 Mar 2005
Posts: 667

Hi, Hafiz.

I really like this effort! If all the statements were accurate (and they may be but I'm not sure), then this could be very useful.  I will try to check for you by duplicating your reasoning process.

In CASE NO 1:

If SPI < 1, then something has slipped.

" i) TF = 0 (It means on schedule but delay in non-critical activities)"

Okay. If TF of activities with lowest TF = 0, it means nothing has yet slipped beyond its float, but conceivably many paths could be burning up their float much too fast. This would be represented by a float burn index (FBI) that is too great and can threaten delay long before it happens. For example, If we have a one year project, but after two months five separate paths have burned up all their float and still have lots of duration left, we are headed for delay even if nothing has negative float. OTOH, if no path is exceeding its acceptable FBI, we are probably okay even if the SPI < 0.

" ii) TF > 0 (It means ahead of schedule but delay in non-critical activities)"

Yes. Good. But again, the FBI is crucial -- the delay in non CP activities may be imminently threatening to delay the project.

" iii) TF < 0 (It means behind the schedule and delay in both critical and non critical activities)"

Not necessarily. The delay may be all on the CP activities. In fact, the non-CP activities may even be ahead of schedule, or performed out-of-sequence in an attempt to keep the SPI from looking worse than it does! The best indicator is how negative is the total float.

in CASE NO 2

If SPI = 1, then either nothing has slipped, or whatever has slipped is precisely offset by other activities being completed early, and perhaps out-of-sequence.

" i) TF = 0 (On schedule) "

Yes, on schedule on the CP. But off the CP, things may have slipped, but the slippage has been offset by early (and perhaps OOS) performance of other activities of sufficient budget to offset the slippage. Again, what is the FBI of the slipped paths?

" ii) TF > 0 (It means ahead of schedule but critical activities are on schedule) "

CP activities might also be ahead of schedule. As in i), off the CP things may have slipped, but the slippage has been offset by early (and perhaps OOS) performance of other activities of sufficient budget to offset the slippage. Again, what is the FBI of the slipped paths?

" iii) TF < 0 (It means non-critical activities on schedule but critical activities are delayed) "

As with CASE NO 1, not necessarily. The delay may be all on the CP activities and the non-CP activities may be ahead of schedule, or performed out-of-sequence, in an attempt to keep the SPI from looking worse than it does! The best indicator is still how negative is the total float.

In CASE NO 3

If SPI > 1, then something has been performed ahead of schedule, but we don't know what until, as you suggest, we check total float.

" i) TF = 0 (It means on schedule) "

It means on schedule on the CP, but some non-CP work could be ahead of schedule and some behind. Again, it's important to check the FBI of anything that has slipped.

" ii) TF > 0 (It means bot critical and non critical activities ahead of schedule) "

Again, some non CP work may be ahead of schedule and some behind.  Check the FBI of any path that has slipped.

" iii) TF < 0 (It means non critical activities ahead of schedule but critical activities are delayed)"

YES! This is the classic diagnostic for a project team that is gaming the SPI: an SPI above 1.0 but negative total float showing that the SPI has been goosed by performing non-CP work ahead of schedule (and possibly OOS).

Hafiz, I think this is excellent and it really helped me to think through the process that you laid out! Please do not be discouraged by the places where I pointed out other issues -- I think you may be on to something very useful here!

If I may, I would recommend my new book Managing Projects as Investments: Earned Value to Business Value. Chapters 8 and 9 explain how earned value is designed to work, point out the flaws, and then make recommendations to correct those flaws.

Fraternally in project management,

Steve the Bajan

www.TotalProjectControl.com