Why logic matters

A schedule is a model of how the work will happen. Every model has inputs, and in CPM there are three: the activities, their durations and the logic that ties them together. The dates are outputs.

That distinction matters the first time something changes. In a logic-driven schedule, a late delivery pushes everything that depends on it, and you can see the new finish date immediately. In a date-driven schedule, where someone typed the start and finish of each activity, nothing moves. The schedule keeps showing the old dates until someone notices, and by then the slip has been hidden for weeks.

What a relationship says

A relationship is a statement about the work: B cannot start until A is finished. It can come from different places:

• Physical logic. You can’t paint a wall before it is built.
• Resource logic. The same crew does both jobs, so one follows the other.
• Preferential logic. There is no hard reason, but this is how the team wants to work.

Physical logic is the one to rely on. Resource and preferential logic are valid but softer: when the project is under pressure, they are the first things worth questioning.

Open ends

Every activity except the first should have at least one predecessor, and every activity except the last should have at least one successor. An activity that breaks this rule has an open end:

• An open start (no predecessor) is scheduled at the project start, because nothing holds it back.
• An open finish (no successor) gets the project finish as its late finish, because nothing waits on it.

The second one is the more dangerous. An open finish shows false float: the schedule says the activity can slip for weeks, when in reality other work is waiting on it.

If an activity really has nothing after it, tie it to the finish milestone. That is honest logic: it says this must be done before we are done.

The website launch

In the lab, A1030 Write page content has neither a predecessor nor a successor. CPM starts it on Day 1, before anyone has agreed what the pages should say, and shows 16 days of total float. Once you tie it in (it starts after Gather requirements and must finish before Load content), it starts on Day 4 and its float drops to 7 days. That is the real margin.

Two more problems are hidden in the same schedule: Test and fix has no successor, and Launch waits only for the content load, not for testing. The status bar’s Open ends counter shows how many activities are still missing logic.

Why reviewers care

Owners and auditors check this first. The DCMA 14-point assessment, a widely used schedule health check, flags a schedule when more than 5% of its unfinished activities are missing a predecessor or a successor. A schedule full of open ends can still calculate dates, but nobody should trust its float or its critical path.