Task Estimating

What is estimating in projects?

We make estimates all the time. For example, to estimate how long it takes to get to work, consider

  • how long it will take to get showered and dressed 
  • other considerations such as feeding and walking the dog, making lunch, getting the kids ready, checking in with parents, and how long it will take to drive
  • one-off planned events such as dropping the car off at the garage, garbage day, and appointments
  • required contingency for other risks such as weather, school closures, bus delays, icy sidewalks, and traffic

Estimating effort and duration for tasks in projects is the same as the above analogy.  You think about

  • the required steps
  • the basic amount of time to complete each step
  • the dependencies for the steps
  • the factors that may affect the timing
  • who else needs to be involved and how much time it will take them
  • the risks that could impact our estimates to determine contingencies

We try to be as accurate as possible with our estimates. Sometimes we look to others to see how long it takes, and use that information as a reference point for ourselves. Sometimes we can use estimates and actual duration from previous projects and initiatives.

The thing is, estimates are estimates. They are based on what we know at the time. The more we do it, the better we get at it. No one ever said an estimate had to be perfect.

Why do we estimate in projects?

  • Need to know how long a project is going to take
  • Dependencies exist between projects. It’s important to know when a dependant task can start.
  • Resources are shared between projects, so it’s important to know resource dependencies and related time
  • Want to deliver efficiently and as fast as possible
  • Want to move on to new things

When do we estimate in projects?

Project estimating can occur at different times within the project, in different levels of detail. 

  • the project charter may contain a list of milestones for the project with estimated timelines
  • higher level estimates may be applied to a package of work before the detailed tasks are known, such as within a Work Breakdown Structure (WBS)
  • packages of work may be broken down to a detailed task level with known dependencies where estimates are applied to each task

Who does the project estimates?

The resources doing the actual work should provide the estimate.

The Project Manager may have to assist the resource in determining the estimate by providing appropriate information such as estimates for dependencies, constraints, and availability of other resources who may also be required.

How are estimates for projects done?

Estimates can be done in terms of effort or duration where

  • Effort is the amount of work units required to complete any given task, if the resource focused on the work 100% of their time. Effort may also be referred to as person-hours, person-days, person-weeks, person-months, or even person-years.

  • Duration is the calendar time required to execute any given task and will take into account other priorities the resource is working on. Duration is measured in hours, days, weeks, months, or years.

In order to determine the task duration, the effort required to complete the task must be determined first. Duration can only be calculated once it is understood who will perform the task, how many people are going to perform the task, and whether they are available to perform the task at a reliable level of availability.

At times, there is a minimum duration that must be accounted for.  For example, for a construction project, duration must account for the time it takes for the cement to dry before a subsequent task can be executed.

There are many different methods that can be used for estimating tasks, some of which include

  • Parametric Estimating. The quantity of work to be performed can be divided by the productivity rate.  For example, 1,000 feet of cable needs to be installed.  On average, the person working on it can lay 25 ft/hour.  It will take 40 hours to complete the job.
  • Three Point Estimating. This method uses three different estimates for a task – most likely, optimistic, and pessimistic.  The three numbers are added together and divided by 3 to provide a more accurate estimate. 
  • PERT Estimating. This is similar to the three point estimate using most likely, optimistic and pessimistic estimate. The difference is the most likely estimate is multiplied by 4, added to the optimistic and pessimistic estimates, and divided by 6.
  • experience and historic project information

Similarly, there are different methods for estimating a project delivery date, some of which include

  • Critical Path Method. Using a list of all tasks required to complete the project, the time (duration) that each activity will take to complete, the dependencies between the activities, and logical end points such as milestones or deliverable items, CPM calculates the longest path of planned activities to logical end points or to the end of the project, and the earliest and latest that each activity can start and finish without making the project longer. This process determines which activities are "critical" (i.e., on the longest path) and which have "total float" (i.e., can be delayed without making the project longer).
  • Timeboxing. This technique works opposite to traditional methods where the effort is based on the scope of the project.  In this case, there is a finite amount of time available and the scope that you will deliver is dependant on that time.  You have 1000 resource hours on a project and your team will produce as much scope as possible.  Whatever is completed at the end of that timebox, is the scope. Agile uses this concept.

Estimation tips and strategies