Activities are the actual work tasks that take time to complete and need resources. Milestones are zero‑duration points that mark the completion of a phase or a key result. They don’t add time to the schedule, but they provide clear checkpoints. Milestones are useful because they give a simple, widely understood signal of progress for stakeholders and make the schedule easier to follow by breaking it into meaningful phases. They let you verify you’ve reached a major achievement before moving on, without tracking every single task in detail. In practice, you plan activities to lead up to a milestone; when the milestone is reached, you can review progress and plan the next set of work. The other ideas mix up duration: activities aren’t zero‑duration tasks, and milestones aren’t tasks with duration.

In PERT, the expected duration is found by weighting the three point estimates so the most likely outcome has the most influence. The formula uses the optimistic time (O), the most likely time (M), and the pessimistic time (P) as TE = (O + 4M + P) / 6. The numbers 1, 4, and 1 add up to 6, so this is a weighted average that gives four times more weight to the most probable duration. This reflects the idea that projects are more often closer to the most likely case than to the extremes, while still accounting for uncertainty from the optimistic and pessimistic bounds. As for the assumed distribution, PERT uses a Beta distribution to model durations. The Beta is bounded between O and P and can be skewed toward the most likely value, which aligns with how people perceive project uncertainty. This is why a simple normal distribution isn’t used in PERT, since normal implies symmetry and unbounded tails, which doesn’t fit the bounded, skewed nature of project activity durations. The standard deviations in PERT are typically expressed in terms of the same bounds, often approximated as (P − O)/6, tying back to the same Beta-based modeling. So, the best answer uses the weighted average with four times the most likely duration and assumes a Beta distribution for durations.

Baseline schedule is the approved reference plan the team signs off on, capturing the originally planned start and finish dates, durations, and sequencing. It stays as a stable benchmark until a formal change is approved, and you use it to measure how the project is performing against what was agreed. The progress schedule, in contrast, is the current, updated view of the project’s timeline. It reflects actual progress to date, current status, and revised estimates for remaining work. It shows what’s happening now and what is forecasted to happen given new information, decisions, or delays. In practice, you keep both in PM software: the baseline as the fixed reference you compare against, and the progress schedule as the live plan you use to manage execution and communicate status. If progress diverges from the baseline, you analyze variances, and if the changes are approved, you may re-baseline to reflect the new approved plan. For example, if a task was planned to finish on January 5, the baseline shows January 5. If work is delayed and the forecast now ends January 10, the progress schedule shows that new forecast, while the baseline remains January 5. This distinction is what makes the progress schedule a tool for current planning and forecasting, separate from the original, approved plan.

Milestone charts and network diagrams serve different purposes in project planning: milestone charts highlight key events with target dates to track progress, while network diagrams map the sequence of work by showing task relationships and dependencies. A milestone chart focuses on significant points in time—moments when a deliverable or approval is due. It helps you communicate progress to stakeholders and see whether the project is hitting major dates. It doesn’t show how long tasks take or how they must be ordered; it’s about the calendar milestones you need to reach. A network diagram, on the other hand, is a scheduling tool that lays out activities and the logical relationships between them. It shows which tasks depend on others, the flow of work, and how delays in one activity affect others. This makes it useful for analyzing sequencing, identifying the critical path, and calculating earliest finish dates. That contrast—dates and milestones versus task relationships and dependencies—is what sets them apart. The other options describe different concepts (budgets, communication lines, risks, channels, resources) that aren’t the primary purpose of these two chart types.