Dependencies, link types and lags
A dependency says what must happen before a task can happen. Real plans use four kinds, and a lag on any of them.
These are the eight planning threads the Krong course is built around. The highlighted ones are covered here. What are the planning threads?
The four links
| Link | Meaning | Example |
|---|---|---|
| Finish to start (FS) | B starts after A finishes | Test starts after build finishes |
| Start to start (SS) | B starts after A starts | Training material starts once design starts |
| Finish to finish (FF) | B finishes after A finishes | Testing ends after the last build ends |
| Start to finish (SF) | B finishes after A starts | Old system runs until the new one starts |
Finish to start is the default and the most common. The others let you overlap work honestly, instead of faking it with shorter durations.
Lag and lead
A lag is a wait on a link. Concrete must cure for two days before the next task: that is a finish-to-start link with a two day lag. A negative lag, called a lead, lets a task start before its predecessor ends.
A lag does not have to be shorter than the task. It is a wait between two tasks, not part of either task's own duration, so a two day task can follow a ten day wait. If the lagged task is on the critical path, the project end moves by the same amount and the task stays critical. Tasks off the path gain float, but their durations do not change.
How to type it in Sen
In the Predecessors column, type the task ID, then an optional link type and lag.
| You type | It means |
|---|---|
1.2.1 | Finish to start, no lag |
1.2.1SS+2 | Start two units after 1.2.1 starts |
1.3FF-1 | Finish one unit before 1.3 finishes |
1.4FS+3 | Start three units after 1.4 finishes |
The unit is whatever your plan uses: days or weeks. Sen recalculates the critical path and float straight away.
Why it matters for integrated planning
A dependency is a link between two parts of the work. If the links are wrong, the dates are wrong, and so are the cost profile and the resource loading built from them. Getting them right is the cheapest way to improve the whole plan.
Questions people ask
Which link should I use by default?
Finish to start. Use the others when work genuinely overlaps.
Can a lag be negative?
Yes. A negative lag is a lead. Use it sparingly, because it hides risk.
Does Sen allow loops?
No. A loop has no valid schedule, so Sen shows an error until you break it.
See it in a real plan
Open Sen and explore the Caldera Freight example. Every figure on this page comes from it. Copy it, change a task, and watch the plan move together.
Open Sen, free About the course