P6 Activity Types: The Four Everyone Confuses

Activity Type is a single dropdown on the General tab, it takes one second to set, and almost nobody thinks about it. Then someone asks why the project management activity is 400 days long, why a crew’s duration stretched when nobody changed anything, or why total float looks absurd on half the schedule — and the answer, more often than not, is in that dropdown.

There are five types in Primavera P6: Task Dependent, Resource Dependent, Level of Effort, WBS Summary and Start/Finish Milestone. Two pairs of them get confused constantly. This post is about those pairs.

Task Dependent vs Resource Dependent

Both types describe real work. The difference is one question: whose calendar decides when work happens?

Task Dependent — the activity’s calendar governs. Assigned resources work when the activity calendar says work happens, regardless of their own calendars.
Resource Dependent — each resource’s calendar governs. Every assigned resource works to its own calendar, and the activity’s duration stretches to accommodate whoever is least available.

Resource Dependent lets the least available resource set the finish date. That is sometimes exactly right — and sometimes a mystery you spend a day debugging.

Use Task Dependent by default. In construction, the crew works when the site works. The activity calendar is the honest model, durations are predictable, and a planner reading the schedule can explain every date.

Use Resource Dependent deliberately, when the resource genuinely controls availability: an inspector who attends on Tuesdays, a specialist subcontractor with a different shift pattern, a shared testing rig booked across projects. In those cases the stretch is not a bug — it is the information you wanted.

The bad example

The most common damage I find in audits: someone resource-loads a schedule and sets everything to Resource Dependent, on the theory that “we have resources now”. Durations start moving on their own, nobody can reproduce last month’s dates, and the planner loses the ability to answer the only question management asks: why is this date what it is?

If your resource calendars are all identical to the activity calendar, the two types behave the same — and the moment someone edits one resource calendar, half the schedule shifts for reasons nobody documented.

Level of Effort vs WBS Summary

Both types describe work that has no duration of its own — they derive their span from something else. The difference is what they derive it from.

Level of Effort (LOE) derives its span from its relationships. You link it to real activities, and it stretches between them.
WBS Summary derives its span from its WBS position. It covers the earliest start and latest finish of the activities in its WBS branch, whether or not anything is linked.

Same visual result, completely different mechanism — and completely different failure modes.

When to use LOE

Time-related work that exists because other work exists: site supervision, project management, temporary facilities, dewatering, quality control presence. Costs that accrue per day rather than per unit of work.

The rule that matters: an LOE is only as good as its links. Typically you tie it with a Start-to-Start from the first real activity and a Finish-to-Finish to the last. Get that wrong and you get an LOE that ends in month three of a two-year job, quietly under-reporting your preliminaries every period.

When to use WBS Summary

When you want a bar that always matches a WBS branch and you do not want to maintain links for it. It is convenient — and that convenience is the trap: because nothing links it, nothing warns you when its span changes. Someone re-parents an activity, the summary silently grows by four months, and no scheduling log complains.

The bad examples

LOE used as a real activity. If work has its own duration and can drive successors, it is a Task Dependent activity. LOE activities do not drive the schedule; making your commissioning phase an LOE removes it from the critical path and hides the very risk you are trying to show.
LOE with no relationships. It has nothing to span between, and what P6 reports afterwards is meaningless.
WBS Summary as a progress line. Because its dates are derived, claiming physical progress against it tells you nothing about the work underneath.
Either type left in the DCMA count. Both are excluded from proper logic checks for a reason; leaving them in your “missing predecessor” numbers inflates findings and makes the real problems harder to see.

Milestones, briefly

Start and Finish Milestones have zero duration and mark a single point in time: contract dates, handovers, permit approvals, notices to proceed. Two habits worth keeping:

Give every milestone logic on both sides where it makes sense. A contractual completion milestone with no predecessors is decoration.
Do not use milestones as a way to pin dates. That is what constraints do — and constraints have their own set of problems, which is a separate post.

A five-minute check on your own schedule

Group your activity list by Activity Type and look at the counts. In a healthy construction schedule you should see: mostly Task Dependent, a handful of milestones, a small number of deliberately placed LOEs, few or no WBS Summary activities, and Resource Dependent only where you can name the reason out loud.

If the mix looks different from that, it is usually not a disaster — but it is always worth knowing why it looks the way it does. If nobody in the team can answer, that is exactly the kind of thing an independent schedule audit surfaces in the first afternoon.