Learn · Scheduling 101
Where a schedule's dates come from: forwards, then backwards
After this subject you can read a computed date as a result rather than a decision: which walk produced it, what would move it, and what would not.
Most of them are worked out for you
You give a schedule a date to start from and four things about the work: the pieces, how long each takes, how they are tied together, and which days your job works. It works most of the dates out from those. Most people have never watched that happen, and it is why a schedule can answer a question a printed picture of one cannot.
The arithmetic behind it
CPM is the arithmetic that turns a start date, durations, links and a calendar into the dates a schedule shows. It is not software and not a Brikt idea: it has been used in scheduling tools since the late 1950s. Those three letters are what schedulers say when they mean it, and the name behind them has a lesson of its own further on.
Why it matters — when a scheduling tool hands you dates, this is usually the arithmetic behind them.
Walking it forwards
The first walk begins at the date the job starts. An early start is the soonest a piece of work can begin — once every tie into it allows, and once any bound that pushes it later allows too. Add the duration, walking the project calendar, and you have the soonest it can end. That walk is the forward pass.
Then walking it back
The second walk begins at the finish and moves back through the same work, answering a different question: how late may this piece run before something gives — the project finish, or a bound set on the work itself? That walk is the backward pass. Between the two, every piece ends up with TWO answers rather than one date.
Why it matters — a bar on a chart shows one of those two answers. What sits between them is the next lesson but one.
Change one thing and it runs again
Make a piece of work longer, add a tie, move the start, and both walks run again over the whole schedule as part of the same edit. You do not ask for it and you do not wait for it. Some dates move and some do not, and which is which depends on the work rather than on where a row sits in the list.
Unless somebody set a date by hand
One thing CAN override the arithmetic: a date a person has fixed. Brikt's grid says so on the column itself — a start is computed from predecessors, unless a bound has been set. When one of those actually bites, that edge stops following the logic. The next lesson is about those bounds, including the ones that are not biting today.
What it cannot work out for you
The arithmetic is only as honest as what you hand it. It does not know how long the work really takes, which pieces are genuinely tied to which, or which days your crews are on site — those three answers are yours. What it does is work the dates out from them, and work them out again the moment one of the three changes.
Why it matters — a schedule nobody argues about is usually a schedule nobody has checked these three answers on.
Reading a date across a table
Ask
Which piece of work is holding this start?
Instead of
Why is this one so far out?
Ask
What changed, to move that finish?
Instead of
The dates moved again.
Ask
Was that date worked out, or set by hand?
Instead of
The schedule says the fourteenth.
What to remember
- CPM is the arithmetic that turns a start date, durations, links and a calendar into dates.
- The forward pass walks from the start and finds the soonest each piece can go.
- The backward pass walks from the finish and finds the latest each one may run.
- Between the two, every piece of work has two answers, not one date.
- A date set by hand can override the arithmetic for that edge — when it bites.
Watch one change travel
Open a project, make one piece of work a day longer, and read what happened to the work after it. Note which rows moved and which did not — either set may turn out to be empty, and why it is empty is the interesting part. For each row that did not move, say out loud why. If you would rather do this on your own job, bring a schedule to a demo and we will walk it together.