Why doesn't SF relation work ?

[sang]

I've found that SF relation did not work at all.
For example,
If I were going home, through two routes, and I want to know the earlier
time to
start heater.
I linked "start heater" activity with "route A" and "route B" with SF options.
I think it will inform me earlier time to start heater.
But "start heater" activity is just before later home arrival time.
Is it a program bug or do I think wrong ?

 

[Jan De Messemaeker]

Hi,

If I understand you correctly you should link Route A and Route B Finish to
the Start of Start heater, thus FS not SF.
HTH

 

[sang]

Heater must be started before arrival home to make home warm.
And it is not possible to predict which route is faster.
So, SF relation seems to be appropriate option because it tells us time to
start the heater before we get home.
In this case, if predecessor were 2(route A and route B), and successor were
linked via SF relation, successor's start time is just before late arrival
time.
I think it should be just before earlier arrival time.
Is it wrong ?
Or is any other method to calculate this problem ?

 

[sang]

Hi,
Thank you for your kindest reply.
But, I am still not clear about the SF relations.
You said that the successor task cannot finish before the
predecessor task starts.
When the predecessors are more than two, the successor is attached just
before the latest starting predecessor.
I think this is wrong. I think the successor must be attached just before
the earliest starting predecessor.
Do I think wrong ?
I'll wait for your kindest reply.

 

[Jan De Messemaeker]

Hi,

When i say that the successor cannot finish before the predecassor starts,
that is true for all predecessors.
So the successor cannot finish before ALL the predecessors have started, so
indeed it cannot finish before the LATEST one has started.

I'm afraid Network planning (not just MS project, Pert planning in general)
doesn't use the logic you expect.

HTH

 

[TA]

Let me make sure I get this straight:
SF (Start to Finish)

I use it to link tasks to a predecessor like an Inspection.
The WBS doesn't necessarily have to be the actual sequence (i.e. This
comes before that, and that comes before the next thing)

So I use SF to link the end of a task to the beiginning a an
"Inspection" task that might actually be ten lines above the successor.


Am I abusing the spirit and integrity of the SF?

 

[Jan De Messemaeker]

Hi,

This is not a matter of lines. That is irrelevant.
The point is that with an SF link the successor cannot finish BEFORE the
start of the predecessor so ikf there are two such predecessors the
successor cannot finish before the LATEST start of the two (because of the
combination of the NOT BEFORE relations).
What you use it for is OK to me as long as you know how it is calculated

 

[Steve House [Project MVP]]

It might help to think of the predecessor task as being the "controlling"
task" while the successor task is the "controlled" task. The predecessor
does not necessarily always come before the successor task in the time
sequence although it does most of the time, rather the predecessor is the
task out of the pair who's timing controls the timing of the other,
regardless of which one it is that is actually done first. A SF link says
that the start time of the predecessor determines the time that the
successor task should finish. An example of one is setting up a dining room
for a banquet. We don't want to do it too early or the ice in the water
glasses and the b utter, etc will melt. We don't want it done too late
because we don't want staff still moving tables etc when our guests start to
arrive. So "Seat guests" is the predecessor, "Set up the room" is the
successor, and the link is SF so the start of the meal seating determines
the time by which we must finish the setup of the room.

In your heater example, I'd say Trip A and Trip B are *both* predecessors to
Heat House, the duration of Heat House is the length of time it takes to
come up to temperature after the heater starts, and the links from both
predecessors are FF so the heatup task finishes at the time the earliest of
the two possible arrivals home occurs.

 

[Steve House [Project MVP]]

Actually made a mistake in my previous post - You could use a SS link to
get the heatup to start when you start home or you could use a SF link so
the heatup finishes by the time you leave for home.

 

[Joaquim Amado Lopes]

Greetings.

Heater must be started before arrival home to make home warm.
And it is not possible to predict which route is faster.
So, SF relation seems to be appropriate option because it tells us time to
start the heater before we get home.
In this case, if predecessor were 2(route A and route B), and successor were
linked via SF relation, successor's start time is just before late arrival
time.
I think it should be just before earlier arrival time.
Is it wrong ?
Or is any other method to calculate this problem ?

As I understood it, you want the house to be warm before you get home
and you don't know exactly when you get home because you can take one
of two different routes. Is this it?

If it is, my solution would be:
Task 1 - Route A (longer) Predecessors: 3FF; 2SS
Task 2 - Route B (shorter) Predecessors: 3FF;
Task 3 - Heat house As late as possible

Notes:
1. house must be warm at the earlist arrive time;
2. don't want to start the heater sooner then necessary (to save
fuel/electricity/...) so task 3 must start as late as possible;
3. house must be warm *before* you arrive at home so the predecessor
is task 3, not the routes;
4. need to make sure the shorter route isn't pushed to the end of
the project (because task 3 is as late as possible), therefore the 2SS
precedence on task 1.

Hope it helps.

Take care,
Joaquim Amado Lopes