Learn · Scheduling 101
Why order matters: what a predecessor is, and the four ways work ties together
After this subject you can say what a predecessor is, tell the four kinds of tie apart by the two ends each one joins, and read a lag as the offset it describes.
The part a printed plan leaves out
A printed plan says the drywall starts on the eighth. It does not say what that date is standing on. Move the inspection and the eighth may be wrong — or may not, if something else is holding it — and the only person who can tell you is whoever drew the sheet. That reasoning has to live in the schedule.
What has to happen first
A predecessor is the piece of work a tie runs FROM; the piece it runs to, whose tied end it constrains, is that predecessor's successor. Which end of each is tied is the next card. The grid names its two columns after the ordinary case: activities that must happen before this one, and activities that depend on this one.
Why it matters — the same tie, read from either end. One fact, two columns.
The tie itself, and there are four of them
A link is the tie between two pieces of work: it joins one end of the first to one end of the second, and there are four kinds of it. Schedulers also call it a dependency, or a relationship — one idea under three names. The four kinds differ only in which end of each piece is tied to which.
The ordinary one, and the one you will draw most
Finish to start is what most people mean by "after". With nothing else written into the tie, the first piece finishes and then the second may begin: strip the forms, then patch the slab. It is the one most schedules are mostly made of, and the one to reach for whenever the second job genuinely cannot begin until the first is done.
The other three, and when you want them
Start to start ties two beginnings: with nothing else written into the tie, the second may begin once the first has — a trailing crew following a leading one down a corridor. Finish to finish ties two endings the same way. Start to finish ties the first piece's START to the second piece's FINISH, and is rare outside a shift handover.
Why it matters — pick the kind by which two ends the work actually ties together, rather than by guessing a date.
How the schedule writes them down
In the grid the four are written as two letters each — FS, SS, FF and SF — beside the number of the piece they point at, with any offset after them. The two letters ARE the two ends, in order. So 230FS+2d reads: activity 230's finish, then this one's start, which may not come until two days later.
When the tie carries an offset of its own
A lag is an offset written into a link: the tied end of the second piece may not come until this many days after the first. A negative one, called a lead, moves that floor earlier instead. Steel takes three weeks to arrive whatever the site does meanwhile, and that wait can be written on the tie rather than buried inside the work before it.
Drawing the order, in the trailer
Ask
Which end of this is waiting, and on what?
Instead of
When does this start?
Ask
Is it waiting on the whole thing, or just the beginning of it?
Instead of
Can those two overlap?
Ask
Is that wait real, or are we padding?
Instead of
Give it a couple of extra days.
What to remember
- A predecessor is the piece a tie runs from; the piece it runs to is its successor.
- A link is the tie between two pieces of work: it joins one end of the first to one end of the second.
- Finish to start is the ordinary one; start to start and finish to finish tie beginnings and endings.
- The grid writes them as FS, SS, FF and SF — the two letters are the two ends, in that order.
- A lag offsets the tied end — a delivery, a cure, an approval — and a lead, which is a negative one, moves that floor earlier.
Say what one week is waiting on
Take the ten pieces of work you named in the last lesson and put in what each one is waiting on. Some will be waiting on work from the week before, some on nothing you have written down, and both are real answers. Then move one of them out by a week and read what moved with it — and what did not, which is just as much of an answer. Bring a week of your own plan to a demo if you would rather work through it with somebody.