Lag and Lead
Jacob Lützhøft Christensen s184113
In the field of project schedule management, the terms lag and lead are widely utilised to maintain project schedule by determining possibilities of schedule advancement or delay. A lead is the amount of time a successor activity can be advanced with respect to a predecessor activity.  A lag is the amount of time a successor activity will be delayed with respect to a predecessor activity.  Traditionally the measure of project success has been limited to concepts of time, quality and cost, however these concepts might not provide a full picture of project success.  In the traditional sense of project success, lag and lead has had an important role in fulfilling the need of time management in projects, however the terms of lag and lead are not purely schedule management tools. According to the project management standard PRINCE 2, the status of projects are best measured for one point in time by its key performance indicators (KPI), which should be balanced between qualitative and quantitative measures, leading and lagging indicators, and project inputs and outputs. 
A key perfomance indicator (KPI) is a measure of performance that is used to help an organization define and evaluate how successful it is in making progress towards its organizational objectives.  These indicators are widely used in organisational contexts, to measure, record, predict and improve perfomance. Indicators for future and past performances are time-based indicators which are distinguished by the terms: lag and lead indicators. This means that lag and lead are not limited to scheduling activities in a task sequence format, but can be used as indicators to determine the perfomance of a project's quality and cost at a given time as well.
Lag and lead can be powerful tools both for project schedule management and as key perfomance indicators. In order to ensure or measure project success it is essential to learn how and when to use these terms, and their limitations.
For schedule management lag and lead are mostly used for sequencing activities. Sequence activity is the process of identifying and documenting relationships among the project activities.  The benefit for this is to define a logical relationship between processes, and thereby obtaining an order of activities that is the most efficient path towards the realization of the project, given all project constraints. This order of activities should then be used to formulate a realistic project schedule. For the project schedule to be realistic, the implementation of tools and techniques such as lead and lag times are necessary. Given the uncertainty aspect of projects, usually for a project schedule to be maintained throughout the project's lifetime, lag and lead are used to either advance or delay certain activities in relation to either its predecessor or successor activities. If lag and lead times were determined for all activities before the project commenced the schedule will be unaffected in a situation of a slight unexpected delay.
As key perfomance indicators lag and lead is incredibly co-dependant and can be used as indication for project performance, cost, quality and more.
A lead indicator, is an indicator which is forward looking. Meaning it attempts to show a future perfomance. These indicators can be difficult to determine, as they are prone to subjectivity and error, and can cause debate as to the validity of the lead indicators measured. . Lagging indicators measure historical perfomance, and tends to focus on outcomes. This makes them easy to record, and they are essential for charting progress, however they do not influence the future as lead indicators do. The power of lead indicators is that they have the ability to predict future conditions, and allow you to take preemptive actions in order to achieve strategic goals. However to predict future conditions, it is necessary to use lagging indicators which essentially record current conditions. These indicators can in combination be applied to determine trends and record the degree of success of pursuing these trends.
Using Lag and Lead as Key Performance Indicators
From the british standard for project management "Prince 2", the definition of lead and lag indicators is: 
- Lagging indicators measure performance that follows events, and allow management to track how well actual performance matches that which was expected. An example could be the number of unexpected errors reported after a particular software release .
- Leading indicators measure progress towards events, and allow management to track whether it is on course to achieve the expected performance. An example would be the persistent failure of a supplier to meet quality requirements early in the project.
The Strategy Map and Balanced Scorecard
Across the perspective of the balanced scorecard, lag and lead indicators can be found.  This shows that the leading and lagging indicators are all a matter of perspective. These can be used program and portfolio management to indicate a future performance, or record past performances and learn from those. For project management, these indicators can also be used to determine the degree of success recorded, or obtainable by a given project. However, the specific indication and how to use them is always a matter of the perspective. This is why the balanced scorecard showcases these key performance indicators well. From figure 1 the perspectives of a balanced scorecard is visualised and made into a strategy map. Kaplan and Norton argued that the leading indicators of one perspective lies in the perspective below. For example:
- In this case, a leading indicator for finances would be the customers. These customers could be an indication of end-of-year profits for an organisation.
- The customers could however in the same case be a lagging indicator for the sales team, as they are the recorded number of customers facilitated.
How to use these indicators is very much up for interpretation, as they indicate different scenarios based on the perspective. To use the strategy map you develop the objectives in each perspective and the cause and effect relationship between them. Then you look for measures and indicators of each objective. This gives you measures in each perspective that should monitor the deliver of the strategy’s cause and effect model. These causes and effects can differ widely, and can be interpreted and utilised in project management as well as portfolio and program management.
For project management, instead of just using lead and lag as indicators for the time aspect, leading and lagging can be used other perspectives, such as cost and quality. All project activities indicate and record the expected and past performance of the project in regards to various perspectives. From a project manager's point of view, these indicators and records should be put to use, to determine the course of the project so far, and in the future. This could be done with the strategy map, for various perspectives. This will allow the manager to change various activities and methods in order to best fulfil the scope of the project and ensure project success. What has become clear through research, is the fact that using the combination of lag and lead indicators has proven to enhance business performance overall. 
Using Lag and Lead to sequence activities
When sequencing activities in regards to a project, determining the order of the activities is crucial. Often one starts with determining the logical relationship between project activities. In the PMI standard for project managing the precedence diagramming method consist of 4 different logical relationships or dependencies.  As defined by this standard the four relationships are:
- Finish-to-start (FS) A logical relationship in which a successor activity cannot start until a predecessor activity has finished.
- Finish-to-finish (FF) A logical relationship in which a successor activity cannot finish until a predecessor activity has finished.
- Start-to-start (SS) A logical relationship in which a successor activity cannot start until a predecessor activity has started.
- Start-to-finish (SF) A logical relationship in which a successor activity cannot finish until a predecessor activity has start.
In practice the FS is the most commonly used relationship, and the SF relationship is the rarest. It is possible for two activities to have two logical relationships at the same time, however multiple relationships between the same activities is not recommended. Here, the project manager should determine the relationship with the highest impact. 
This determination of logical relationships between activities is the first step to build a Project Schedule Network Diagram, and it is necessary in order to determine lag and lead times between activities.
Here is an example of the use of lag and lead times from the PMI standard for project management: 
On a project to construct a new office building, the landscaping could be scheduled to start 2 weeks prior to scheduled punch list completion. This would be shown as a finish-to-start with a 2-week leads as shown on figure 3. For lag, a technical writing team may begin editing the draft of a large document 15 days after they begin writing it. This can be shown as a start-to-start relationship with a 15-day lag as shown in Figure 3.
In practice, the activities for figure 3 could not be properly scheduled without lead and lag times. In combination the the logical relationship, the project manager is able to use lead and lag to properly schedule and visualise the project timeline. This greatly improves the opportunity for project success, as all stakeholders and project members will be able to know the sequence of activities, when the Project Schedule Network Diagram is finished.
Project Schedule Network Diagram
A Project Schedule Network Diagram is a graphical representation of the logical relationships among the project schedule activities.  It serves as an important tool for visualisation for the project manager to be able to keep track of activities that has to be completed in order for the project to be completed. The Project Schedule Network Diagram addresses the logical relationships between project activities to show the order of which the activities should be scheduled. It is an overview of the entire project timeline, that can be used for:
- Determining the most important project activities in regards to the time-line.
- Keeping track of the project's development throughout its lifetime.
- Rescheduling activities in accordance with lag and lead times.
- Clarifying the optimal use of lag and lead times and logical relationships between activities to ensure project success.
The benefits of this tool is that the project manager is able to track dependencies and potential bottlenecks, establish workflows and the visual representation can be provided to the stakeholders as well. In this tool lag and lead times are crucial to the development. The project management team should determine the dependencies that may require a lead or a lag to accurately define the logical relationship. The use of leads and lags should not replace schedule logic. Also, duration estimates do not include any leads or lags. 
In large projects however, the use of this method to communicate the schedule is very rare, as the diagram can become very complicated. More often a more aggregated view of the schedule is used (ie. a Gantt-diagram).  In these cases the Project Schedule Network diagram is used more often as a complex document linking the dependencies and sequences to the overall project schedule and duration. 
An estimation of completion time for a project can be determined after the completion of the Project Schedule Network Diagram. Here Lag and lead times are used to decide the Critical Path.
Critical Path Method
According to the PMI standard for project management: The critical path method is used to estimate the minimum project duration and determine the amount of schedule flexibility on the logical network paths within the schedule model.  By performing a backwards and forwards pass analysis together with lag and lead times through the project network, this method calculates the early start, early finish, late start and late finish dates of all activities without any regard for resource limitations.  In figure 5, the longest path is shown with black arrows. This path, since it's the longest, determines the earliest completion time of the given project. This is known as the critical path, and it is often the path with the least amount of float.
Float is not to be confused with lag and lead. The float is defined as: "The total float or schedule flexibility is measured by the amount of time that a scheduled activity can be delayed or extended from its early start date without delaying the project finish date or violating a schedule constraint." Where lag and lead times show the possible delay or advancement in relation to other activities, the float is an indicator of the schedule's flexibility. Therefore it makes sense that the critical path, would be the path with the least float time, as delaying an activity in the critical path would usually extend the project finish date.
To calculate the early start, early finish, late start, late finish and float first, the forward pass is used: 
- Early Start = Maximum (or Highest) EF value from immediate Predecessor(s) + Lag/lead from immediate Predecessor(s)
- Early Finish = ES + Duration
Afterwards the backwards pass is used:
- Late Start = LF – Duration
- Late Finish = Minimum (or Lowest) LS value from immediate Successor(s) - Lag/Lead from Immediate Successor(s)
These numbers are then used to calculate the float time:
- Total Float = LS – ES (it is also calculated by LF – EF)
- Free Float = Lowest ES of successors – EF
Limitations of lag and lead in Sequencing Activities
When using lag and lead for sequencing activities it is important to note, that it is not a method that can be used as a standalone. Lag and lead must be used in combination with other tools, as the lag and lead times in a Project Schedule Network Diagram are negligible without determining the logical relationships between the activities. In other words, there would be no use for them.
Lag and lead times does not include the duration of activities in the project network. It is purely a schedule measure to determine whether or not a given activity can be delayed or advanced. They are necessary and important, however they do not provide further insight to the sequencing of activities other than the time schedule by themselves.
Lag and lead times are also not absolute and can be a subject to change throughout the course of the project, and can be very hard to determine in the beginning of the project. Therefore project network usually needs to be updated as the project progresses. It is of utmost importance for a project manager to keep this in mind, as the consequences for the scheduling can be severe if wrong lag and lead times are used.
Limitations of lag and lead as Key Performance Indicators
Using lag and lead indicators as Key Performance Indicators, it is important to remember that they are dependant on each other. Lead indicators are exciting because they can influence change, but without lagging indicators, leading indicators will encourage tunnel vision, and provide no confirmation that the desired results have been achieved.  Lagging indicators without leading indicators provide no indicators on how to achieve or change results.  Lagging indicators are also very easy to measure, but hard to improve, whereas leading indicators, if relied on too much can distort course of the project. When using lead indicators, it is important to differentiate between trying to predict the future, and searching for tendencies.
Leading indicators alone will show a future that is unreliable. The tendencies achieved from measuring leading indicators should be carefully examined. Exterior factors can influence the indicators, and therefore it is very important for a manager not to get tunnel vision. They are not an accurate representation of the future, but they are however representations of tendencies, that can be pursued. These tendencies have to undergo careful examination in order to determine the best possible way to pursue the desired direction or outcome. In order to reach that destination, lagging indicators are just as important, to track whether the management is on the right path. Knowing the limits of the tools is crucial for the project manager to use them correctly.
Project Management: A guide to the Project Management Body of Knowledge (PMBOK guide) 6th Edition, Project Management Institute, Inc. (PMI), 2017
This book is Project Management Institute's standard for project management. This has provided the definition of lag and lead in the context of scheduling activities. It has also provided an overview of various methods and tools that can be used for scheduling activities in combination with lag and lead. There are additional tools, to the ones included in this article, described in this standard.
Project Management: "Managing Successful Projects with PRINCE2" 6th Edition, AXELOS and Office Cabinet, 2017
This book has provided the definition for KPI's and lagging and leading indicators. It is the british standard "Prince 2" for project management. The material that this book has to offer on this subject is scarce. It is important to note that this standard's definition of other topics included in this article differ slightly from PMI's standard, which is the one that has been used for this purpose.
Lead and Lag Indicators https://www.intrafocus.com/lead-and-lag-indicators/
An article on lead and lag indicators from Intrafocus. This article provides general definitions on the terms as well as a basic understanding of the fundamentals behind leading and lagging indicators, as well as some of their limitations.
Leading indicators and lagging indicators – making sense of them, Phil Jones (2017) https://www.excitant.co.uk/leading-and-lagging-indicators-making-sense-of-them/
An article by Phil Jones for Excitant.co.uk. This article provides further insight to the use and definitions of lagging and leading indicators, as well as specific examples for their uses, in organisational contexts.
Project Schedule Network Diagram: Definition | Uses | Example, Project management info https://project-management.info/project-schedule-network-diagram/#2-what-is-a-project-network-diagram-used-for
This article provides some clarification of the uses and the definition of the project network diagram. It also provides various examples and figures, one of which has been used in this article.
Kramer, S. W. & Jenkins, J. L. Understanding the basics of CPM calculations: what is scheduling software really telling you?, Project Management Institute, Inc. (PMI), 2006 https://www.pmi.org/learning/library/critical-path-method-calculations-scheduling-8040
An excerpt of a paper presented at the Project Management Institute in 2006. This paper provides the method of calculation the different variables in the critical path method, that is included in this article.
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 Project Management: A guide to the Project Management Body of Knowledge (PMBOK guide) 6th Edition, Project Management Institute, Inc. (PMI), 2017
- ↑ 2.0 2.1 2.2 2.3 Project Management: "Managing Successful Projects with PRINCE2" 6th Edition, AXELOS and Office Cabinet, 2017
- ↑ 3.0 3.1 3.2 3.3 Lead and Lag Indicators, Intrafocus, Last visited: 14/02/21
- ↑ 4.0 4.1 Leading indicators and lagging indicators – making sense of them, Phil Jones (2017), Last visited 28/02/21
- ↑ 5.0 5.1 5.2 Project Schedule Network Diagram: Definition | Uses | Example, Project management info, Last Visited 28/02/21
- ↑ Kramer, S. W. & Jenkins, J. L. Understanding the basics of CPM calculations: what is scheduling software really telling you?, Project Management Institute, Inc. (PMI), 2006