Schedule Management and Project Time Management

## Schedule Management Schedule management is critical in construction due to the cascading effects of delays, including cost overruns, inefficient resource use, reduced quality, and lower client satisfaction. A robust schedule is the backbone for coordinating multiple contractors, optimizing resource allocation, and ensuring materials and equipment are available at the right time. Effective schedule management also facilitates better communication among stakeholders and allows for proactive risk management. A study by the Bank of Italy indicates that at least one-third of public works under the PNRR are behind schedule, highlighting the pervasive challenges in maintaining project timelines. ### Origins of Schedule Management Coordinating work over time dates back to early construction endeavors, such as the construction of the Egyptian pyramids and medieval cathedrals. Initially, architects, engineers, and master builders managed civil engineering projects using empirical methods and rudimentary tools. The industrial revolution and Frederick Taylor's scientific management principles led to the development of more systematic approaches, culminating in the Gantt chart in the early 20th century, created by Henry Gantt. ### History - **Henry Gantt (1861-1919):** Introduced the bar chart in 1910, a visual planning tool representing a project's schedule as a horizontal bar chart. Gantt charts provided a simple yet effective way to track project progress and identify potential delays. - **DuPont Company (late 1950s):** Developed the Critical Path Method (CPM), a deterministic scheduling tool identifying the longest sequence of dependent tasks and optimizing project timelines. CPM enabled project managers to focus on critical activities and allocate resources efficiently. - **Sputnik Crisis (1957):** Revived "scientific management," emphasizing the need for advanced project management techniques to achieve ambitious goals. - **Polaris (1958):** Introduced the Project Evaluation and Review Technique (PERT) to acknowledge uncertainty in project schedules, utilizing probabilistic estimates to account for variability in task durations. ### Planning Schedule Management Defines how the project schedule will be planned, developed, managed, and controlled. Key components include the selection and application of appropriate tools and techniques for scheduling, defining roles and responsibilities, establishing schedule baselines and performance metrics, and outlining change management procedures. A well-defined plan ensures that all stakeholders are aligned and that the schedule is realistic and achievable. #### Steps: 1. Defining Project Activities 2. Sequencing Project Activities 3. Estimating Activity Durations 4. Developing the Project Schedule 5. Controlling the Project Schedule ### Defining Project Activities Involves identifying and documenting specific actions to produce project deliverables. Clear and comprehensive activity definitions are essential for accurate scheduling and resource allocation. #### Steps: - Decompose project work packages into manageable tasks using a Work Breakdown Structure (WBS). The WBS provides a hierarchical decomposition of the project scope, ensuring that all necessary activities are identified. - Create an activity list and define dependencies. The activity list should include detailed descriptions, resource requirements, and estimated durations for each task. - Include milestones and deliverable checkpoints to track progress and ensure that the project stays on schedule. Milestones should be specific, measurable, achievable, relevant, and time-bound (SMART). ### Sequencing Project Activities Establishes the logical order of tasks based on dependencies. Proper sequencing ensures that activities are performed in the correct order, minimizing delays and maximizing efficiency. #### Types of Dependencies: - Finish-to-Start (FS): An activity needs to finish before a successor can start. - Start-to-Start (SS): An activity needs to start before its successor starts. - Finish-to-Finish (FF): An activity needs to finish before its successor finishes. - Start-to-Finish (SF): An activity needs to start before its successor finishes. #### Techniques: - **Precedence Diagramming Method (PDM):** Represents project activities on a graph, mainly Activity on Node (AON) graphs, illustrating the sequence of tasks and logical relationships. PDM provides a visual representation of project dependencies, facilitating communication and coordination among team members. - **Dependency determination and leads/lags analysis:** Involves identifying how tasks relate to one another. Lead Time allows overlapping tasks, accelerating the project schedule, while Lag Time is the delay inserted between tasks, providing necessary buffer time. ### Project Graphs Graph theory models pairwise relations between objects, consisting of vertices (nodes or points) connected by edges (arcs or lines). Graphs can be undirected or directed, depending on whether the relationships between objects are symmetrical. Project graphs are used to visualize and analyze project schedules, identify critical paths, and optimize resource allocation. #### Types of Network Diagrams: - Activity on Arrow (AOA) - Activity on Node (AON) ### Activity-on-Arrow (AOA) Diagrams In AOA diagrams, the activity name and duration are specified on the arrow between two nodes. A node represents a specific, definable achievement with zero duration and nil resources. AOA diagrams were historically significant but have declined in use with the advent of computer-based scheduling tools that offer more flexibility and ease of use. #### AOA Diagrams Rules - All activities leading into a node must be completed before activities following the node can start. - Activity arrow length or shape has no meaning. - Two nodes cannot be connected by more than one activity (arrow). - There can be only one starting node and only one ending node. ### Bar (Gantt) Charts Bar charts display activity start and end dates, expected durations, and dependencies. They are easy to read and widely used in management presentations to provide a high-level overview of the project schedule. However, dependencies can make Gantt charts difficult to read in complex diagrams, as the logical links are drawn by lines that descend vertically, often resulting in overlapping lines. ### Finish to Start - An activity needs to finish before a successor can start. ### Start to Start - An activity needs to start before its successor starts. ### Finish to Finish - An activity needs to finish before its successor finishes. ### Time-Location Diagram A method to graphically present a schedule for linear projects like pipeline construction, railways, bridges, tunnels, roads, and transmission lines. Activities are displayed along a time axis and a distance axis, showing position, direction, and rate of progress. Different activity types are differentiated by color, fill type, line type, or special symbols. This visualization aids in understanding the spatial and temporal aspects of linear projects. ### Estimating Activity Durations Predicts the time each activity will take to complete, based on available resources, historical data, and expert judgment. Accurate duration estimates are crucial for creating realistic project schedules and allocating resources effectively. #### Methods: - **Expert Judgment:** Relies on insights and experience of subject matter experts who have worked on similar projects. This method leverages the knowledge and expertise of experienced professionals to estimate activity durations. - **Analogous (Top-down) Estimating:** Uses historical data from similar projects to estimate the duration of current activities. This method is useful when detailed information is not available, but it relies on the assumption that past projects are comparable to the current one. - **Parametric Estimating:** Applies statistical relationships between historical data and project parameters, such as size, complexity, and location. This method is more accurate than analogous estimating but requires reliable historical data. - **Three-Point Estimating (Optimistic, Pessimistic, Most Likely):** PERT uses three estimates to calculate a weighted average duration, accounting for uncertainty. This method provides a more realistic estimate than single-point estimates. - **Bottom-Up Estimating:** Aggregates estimates from smallest work packages to estimate the total duration of larger activities. This method is the most accurate but also the most time-consuming. ### Developing the Project Schedule Combines activities, durations, resources, and dependencies to create a project schedule. The schedule serves as a roadmap for executing the project and tracking progress. #### Tools and Techniques: - Critical Path Method (CPM) - Program Evaluation and Review Technique (PERT) - Schedule Compression (Crashing, Fast-Tracking) - Resource Leveling and Smoothing ### Critical Path Method A project scheduling technique that identifies the longest sequence of dependent tasks. It determines the minimum project duration by highlighting tasks with zero slack. CPM is essential for managing project timelines and ensuring on-time completion. #### Key Concepts: - **Critical Path:** The sequence of tasks that directly affect the project completion time. Any delay in critical path activities will delay the entire project. - **Float/Slack:** The available time to delay non-critical tasks without affecting the overall schedule, based on Earliest Event Time and Latest Event Time, which can be computed from a graph (AOA). ### Earliest Event Time and Latest Event Time - **Earliest Event Time (EET):** The earliest time an event can begin. 1. Select all activities entering the event. 2. Sum the activity's duration and the EET of its initial event for each activity. 3. Select the highest EET obtained. $T*{EETi} = 1$ $T*{EETj} = T*{EETi} + D*{i,j}$ - **Latest Event Time (LET):** The latest time an event can begin without affecting the project schedule; it is computed by working backward from the terminal node. 1. Consider all activities leaving an event. 2. Subtract each activity's duration from the LET of its terminal event. 3. Select the smallest LET obtained. $T*{LETterminal} = T*{EETterminal}$
$T*{LETi} = min(T*{LETj} - D_{i,j})$ ### Forward Pass - Graph is traversed from left to right, and the maximum time at nodes with more incoming activity is considered. ### Backward Pass - Graph is traversed from right to left, and the minimum time at nodes with several outgoing activities is considered. ### Slack/Float Time - The difference between the LET and EET represents how much additional time is available for slippage which is given by: $Slack Time = T*{LET} - T*{EET}$ - The path on the directed graph where the slack time is zero is the Critical Path. - Float (for a given activity): The available time to delay non-critical tasks without affecting the overall schedule. ### Schedule Compression - **Fast-Tracking:** Re-sequencing project activities to perform tasks in parallel, increasing the risk of rework. This technique requires careful coordination and communication to minimize potential issues. - **Crashing:** Adding extra resources to tasks on the critical path, increasing project costs. Crashing is typically used when the project is behind schedule and needs to be completed as quickly as possible. ### Crash Cost and Duration Each task has a nominal duration and cost. Adding resources to the task can reach a minimum crash duration and a maximum crash cost. The cost of accelerating a task of one unit of time is: $(C*{crash} - C*{nominal}) / (T*{nominal} - T*{crash})$ - The cost of gained time ### How to Crash a Project Given an AoA diagram: - The cost of gaining time (Cgt) of all activities is computed. - One at a time, we reduce the durations of individual activities starting from the one with the lowest Cgt to the crash duration (if possible) modifying the cost. - Project duration and project cost are recalculated. - Never change the critical path (do not reduce task duration down to the crash time if the CP changes). ### Project Optimisation Results The project time and cost optimisation process saves time (contractual constraints) and optimises costs. ### Crashing – Some Considerations - The CP is the "bottleneck route." - Shortening or lengthening tasks on the critical path directly affects project finish. - The duration of "non-critical" tasks is irrelevant (as long as the CP doesn't change). - “Crashing” all jobs is ineffective, focus on the few % of jobs that are on the CP - “Crashing” tasks can shift the CP to a different task, previously non-critical tasks can become critical. Shortening of non-critical tasks can also shift the critical path. - Shortening tasks – technical and economical challenge (How can it be done?) ### Resource Leveling A technique used to resolve resource conflicts or overloads by adjusting project activities, often involving changing task start and end dates to keep resource usage within limits. Resource leveling aims to optimize resource utilization and prevent overallocation. #### Objective: - To balance resource demand with resource availability. - To ensure that no single resource is over-allocated or under severe pressure. #### Primary Approach: - May extend the project duration if necessary. - Tasks are rescheduled according to priority. ### Resource Smoothing A technique that aims to adjust the resource schedule without changing the project’s critical path or overall duration. Resource smoothing seeks to minimize fluctuations in resource usage while adhering to project deadlines. #### Objective: - To minimize fluctuations in resource usage. - To maintain a more consistent or “smooth” resource demand profile. #### Primary Approach: - Activities are rescheduled within the available “float” or “slack.” - The overall project end date typically remains unchanged. ### Key Differences Between Resource Leveling and Smoothing | Aspect | Resource Leveling | Resource Smoothing | | ----------------------------------------- | ----------------------------------------- | ----------------------------------------- | | | | | | Goal | Resolve over-allocation | Maintain consistent resource usage | | Impact on Duration | May extend the project duration | Typically keeps overall project duration | | Use of Float | Tasks may be shifted beyond float | Tasks shifted only within available float | | Priority | Ensure resource limits not exceeded | Even out peaks and valleys | ### Program Evaluation and Review Technique The PERT (Program Evaluation and Review Technique) is a tool for analysing the tasks involved in completing a project, especially the time needed to complete each task. It incorporates uncertainty by making it possible to schedule a project while not knowing precisely the details and durations of all the activities. PERT is particularly useful for projects with high levels of uncertainty and complexity. ### Activities duration estimate The technique requires three duration estimates for each individual activity, as follows: - Optimistic time estimate ($t_o$): shortest possible time in which the activity can be completed, assuming that everything goes perfectly - Realistic time estimate ($t_m$): most likely time in which the activity can be completed under normal circumstances - Pessimistic time estimate ($t_p$): longest possible time the activity might require, assuming a worst-case scenario Based on the three estimates, a weighted average ($D_{me}$ median duration) and variance is calculated for each activity duration as a measure of the average duration and the corresponding variability, respectively. $D*{me} = (4t*m + t*o + t*p)/6$
$s = (t*p - t*o)/6$ ### Probability to finish within the contractual timeline - The ratio between the float of the last event (i.e. the difference between the contractual duration and the programme duration computed using median durations) and the magnitude of the range of uncertainty (measured as the square root of the sum of the standard deviations of the activities on the critical path) is assumed to follow a normal distribution and is denoted by Z. $Z = (T*{pf} - T*{minf}) / (∑*{i∈CP} σ*i^2 )^{0.5}$ Where: - $T_{pf}$ is the contract duration - $T_{minf}$ is the duration computed from the graph - $σ_i$ standard deviation. ### Guaranteed program duration Instead of using the duration computed in the graph ($T_{minf}$) as contract time a longer one is taken ($T_{pf}$). The longer duration is determined adding a quantity of time proportional to the cumulative curve of the random variable Z. This allow to select a contract duration with a given probability of success. $T*{pf} = Z * (∑*{i∈CP} σ*i^2 )^{0.5} + T*{minf}$ ### PERT limitations PERT is based on restrictive assumptions: - all tasks have duration uncertainty distributed according to the same statistical function; - the critical path never changes. Both assumptions are almost never true in real-life project. ### Monte Carlo Simulations Monte Carlo simulations constitute the most commonly employed method for estimating the influence of uncertainties in duration estimates on a program’s overall timescale, as depicted by graph-based or GANTT representations. These simulations rely on numerous iterations in which the program duration is recalculated, with task durations in each iteration drawn from statistical distributions.