4 Schedule Planning and Control

Define activities and estimate durations

A project schedule translates approved scope into a time-based plan: the work to be done, its order, expected durations, and planned start and finish dates. A useful schedule is built from complete activities, realistic estimates, and logical dependencies. Once approved as a baseline, it provides a reference for measuring progress and evaluating proposed changes.

Start from the project deliverables and break the work into activities that can be assigned, estimated, and tracked. Define each activity clearly enough that the team can tell when it is complete. For every activity, identify the resources, working calendar, assumptions, and constraints that affect its duration.

Estimate durations using relevant experience, expert judgment, or comparable past work. Treat estimates as forecasts based on stated assumptions; uncertainty in individual activities can affect the project finish date. When uncertainty matters, record a range rather than false precision.

Three-point estimates

A three-point estimate uses an optimistic duration $O$, a most likely duration $M$, and a pessimistic duration $P$. One common weighted estimate is:

For $O=2$ days, $M=4$ days, and $P=8$ days, the expected duration is $\frac{26}{6}$, or about $4.3$ days.

Sequence activities and model dependencies

Identify which activities must precede others and which can proceed in parallel. A network diagram makes this logic visible. The most common relationship is finish-to-start (FS): the successor cannot start until the predecessor finishes. Other relationships include start-to-start, finish-to-finish, and start-to-finish.

A lead allows overlap, while a lag requires a waiting interval. Use leads and lags only when they represent real work or waiting, not to conceal missing activities or unrealistic logic.

For example, a team may need to complete a design before building a product, while procurement of standard materials can happen at the same time as design. Capture genuine dependencies, including approvals, deliveries, and external decisions. Missing or incorrect links can distort the schedule and its calculated float.

Calculate the critical path and float

Combine activities, duration estimates, dependencies, calendars, and resource availability to calculate planned dates. A network schedule is often developed with the critical path method (CPM).

1. In a forward pass, calculate each activity’s earliest start (ES) and earliest finish (EF), moving from the beginning of the network. For a simple finish-to-start network, $EF=ES+\text{duration}$; an activity’s ES is the latest EF among its predecessors.

2. In a backward pass, calculate latest finish (LF) and latest start (LS), moving back from the required completion date. For a simple finish-to-start network, $LS=LF-\text{duration}$; an activity’s LF is the earliest LS among its successors.

3. Calculate total float as $LS-ES$, equivalently $LF-EF$. Float is the time an activity can slip without delaying the calculated project finish, given the schedule logic and constraints.

Activities on the critical path have zero total float in the basic example. The critical path is the longest-duration path through the network and determines the earliest possible project completion under the modeled logic. A delay to a critical activity can delay the finish unless the team takes effective action. The path can change as actual progress, remaining durations, or dependencies change.

Worked network example

The activities and immediate predecessors are:

• $\mathrm{A}$: confirm requirements; duration $2$ working days; no predecessor.

• $\mathrm{B}$: complete design; duration $3$ working days; predecessor $\mathrm{A}$.

• $\mathrm{C}$: procure kit; duration $4$ working days; predecessor $\mathrm{A}$.

• $\mathrm{D}$: build prototype; duration $5$ working days; predecessors $\mathrm{B}$ and $\mathrm{C}$.

• $\mathrm{E}$: test prototype; duration $2$ working days; predecessor $\mathrm{D}$.

Activities $\mathrm{B}$ and $\mathrm{C}$ can proceed in parallel after $\mathrm{A}$. The paths are $\mathrm{A-B-D-E}$, with duration $12$ days, and $\mathrm{A-C-D-E}$, with duration $13$ days. Therefore, the critical path is $\mathrm{A-C-D-E}$, and the earliest planned completion is $13$ working days after the project starts.

The calculated early and late dates and total float are:

• $\mathrm{A}$: $ES=0$, $EF=2$, $LS=0$, $LF=2$, total float $0$.

• $\mathrm{B}$: $ES=2$, $EF=5$, $LS=3$, $LF=6$, total float $1$.

• $\mathrm{C}$: $ES=2$, $EF=6$, $LS=2$, $LF=6$, total float $0$.

• $\mathrm{D}$: $ES=6$, $EF=11$, $LS=6$, $LF=11$, total float $0$.

• $\mathrm{E}$: $ES=11$, $EF=13$, $LS=11$, $LF=13$, total float $0$.

Activity $\mathrm{B}$ has one day of float in this network; it could slip by one day without changing the $13$-day finish, assuming all other estimates and logic remain valid. This is a network calculation, not a guarantee: resource limits, calendars, risks, and imposed dates can affect the executable schedule. Check a credible schedule for complete activities, sound logic, and realistic resource assumptions.

Track performance and control schedule changes

After the responsible stakeholders approve the schedule, preserve it as the schedule baseline. Regularly record actual starts and finishes, remaining durations, and forecast dates, using a consistent status date. Compare current forecasts with the baseline to see whether milestones or the project finish are at risk. A baseline is a comparison point, not a reason to hide a forecast delay by silently changing planned dates.

When a change is proposed or a variance emerges:

1. Describe the change or cause, and identify affected activities, dependencies, assumptions, and milestones.

2. Assess effects on the critical path, project finish, resources, cost, risks, and deliverables. Check whether near-critical paths could become critical.

3. Present options and trade-offs. Possible responses include resequencing work where logic permits, reducing scope through an approved decision, adding resources, or using overtime. Adding resources or overlapping tasks may increase cost or risk and may not reduce duration.

4. Obtain approval through the project’s agreed change-control process before revising the baseline. Document the decision, rationale, and revised assumptions.

5. Update the working schedule, communicate the new forecast, and continue tracking against the approved baseline or formally revised baseline.

Schedule risk analysis can help assess uncertainty in completion dates and identify activities that deserve attention, including activities that may become critical under different risk scenarios.