Gantt chart Study Guide

📖 Core Concepts Gantt chart – a bar chart that visualizes a project schedule; tasks on the vertical axis, time on the horizontal axis. Bar width – shows each task’s duration; the bar’s start / end points mark planned start and finish dates. Work Breakdown Structure (WBS) – the hierarchy of summary and terminal tasks that the chart displays. Dependency (precedence) lines – arrows or connectors indicating “task A must finish before task B can start.” Today line & percent‑complete shading – modern visual cues for current schedule status. Task time estimates – optimistic (O), most‑likely (M), pessimistic (P) values used in PERT‑style calculations. Expected time (TE) – weighted average of O, M, P: \(\displaystyle TE = \frac{O + 4M + P}{6}\). Early‑Start scheduling – schedule each activity as soon as all its predecessors finish; maximizes float. Float (slack) – the amount of time an activity can be delayed without delaying the whole project. --- 📌 Must Remember Axes: vertical = tasks; horizontal = time. TE formula: \(\displaystyle TE = \frac{O + 4M + P}{6}\). Early‑Start → maximum float; opposite is Late‑Start (minimizes float). Gantt vs. CPM network: Gantt is easy to read; CPM network shows critical path more clearly. Dynamic Gantt: auto‑updates when dates change; static Gantt does not. Critical Path: the longest‑duration path; determines minimum project length. --- 🔄 Key Processes Build a Gantt chart List all tasks (vertical). Determine time scale (horizontal). Draw a bar for each task: start = earliest start, length = duration. Add dependency arrows where needed. Insert today line and shading for progress. Calculate Expected Task Duration (PERT) Gather O, M, P estimates. Apply \(\displaystyle TE = \frac{O + 4M + P}{6}\). Use TE as the bar length for that task (if using PERT‑based schedule). Early‑Start Scheduling Identify predecessor tasks for each activity. Set start date = max(finish dates of all predecessors). Continue forward through the network to assign all start/finish dates. Determine Float Compute Late Finish (LF) and Late Start (LS) via backward pass. Float = LS – ES (or LF – EF). --- 🔍 Key Comparisons Gantt chart vs. Critical‑Path network diagram Gantt: visual timeline, easy for non‑experts, can get cluttered with many dependencies. Network: emphasizes logical relationships & critical path, clearer for complex projects. Static Gantt vs. Dynamic Gantt Static: hand‑drawn or frozen; requires manual updates. Dynamic: software‑generated; auto‑updates when task dates shift. Early‑Start vs. Late‑Start scheduling Early‑Start: starts tasks ASAP, maximizes float. Late‑Start: delays tasks until needed, minimizes float (used for “just‑in‑time” schedules). --- ⚠️ Common Misunderstandings “Gantt shows critical path.” – Gantt can display dependencies but does not automatically highlight the critical path; you must analyze the network or use a CPM diagram. “Longer bar = more important task.” – Bar length only reflects duration, not priority or risk. “Percent‑complete shading equals actual progress.” – Shading is a planned status indicator; it must be manually updated to reflect real progress. “Dynamic Gantt eliminates all errors.” – If the underlying task data are wrong, the auto‑updated chart will still be incorrect. --- 🧠 Mental Models / Intuition Timeline as a “train track.” Imagine each task as a train car on a track; the car can’t leave the station (start) until the previous car (predecessor) has arrived. Float as “cushion.” Visualize float as a mattress under a task—push down (delay) a little, and the project still stays level. TE formula as “weighted average.” The most‑likely estimate gets four‑times the weight of the extremes, pulling the average toward realistic expectations. --- 🚩 Exceptions & Edge Cases Cluttered dependencies – In very large projects, linked Gantt charts become unreadable; switch to a CPM network or break the project into sub‑Gantts. Non‑sequential start constraints – Some tasks may have “must start on a specific date” constraints that override early‑start logic. Resource‑leveling – Early‑Start may create overallocation; a resource‑leveling step may shift tasks later, reducing float. --- 📍 When to Use Which Use a Gantt chart when you need a quick visual schedule for stakeholders, especially for small‑to‑medium projects with ≤ 30 tasks. Switch to a CPM network diagram for large, highly interdependent projects where identifying the critical path is essential. Choose dynamic Gantt software when the schedule is expected to change frequently (e.g., Agile sprints). Apply the TE formula whenever you have O‑M‑P estimates (PERT style) and need a single duration value for planning. --- 👀 Patterns to Recognize Bar clusters with the same start date → likely share a common predecessor. Long horizontal “float” gaps between bars → indicates possible slack. Vertical “TODAY” line intersecting a bar → spot tasks that are currently in progress; if the bar is only partially shaded, progress may be off‑track. Dependency arrows forming a chain → potential critical path candidate (check total duration). --- 🗂️ Exam Traps Mistaking “longest bar” for “critical task.” The longest duration does not guarantee it lies on the critical path. Using the TE formula without the 4‑weight on M. Some students mistakenly average O, M, P equally; the correct weight is 4 for M. Assuming dynamic Gantt automatically fixes schedule conflicts. The software updates dates but does not resolve logical conflicts (e.g., predecessor finish after successor start). Confusing float with “extra time” given to a task. Float is available delay time; it is not automatically allocated unless the schedule actually slips. ---