Table of Contents for Project Duration Estimation-Tips & Tricks–
- How to Estimate Activity Duration?
- Bottom to Top Estimation
- Top to Bottom Estimation
- Expert Judgement
- Analogous Estimation
- Parametric Estimation
- Three-Point Estimation
The duration estimation of an activity is an important step in making a project schedule. The success of the project schedule lies in the completion of scope work within the time. Thus, the correct time estimation of activity is a critical task to ensure the timely completion of the job.
The correct estimation of duration is also critical in terms of resource planning. The incorrect duration estimate may lead to either scarcity of resource or the wastage of the same. In the said case, either the time is lost or money or both.
The project time estimation tips & tricks include the top six approaches commonly applied in construction, IT, defence, healthcare etc.
How to Estimate Project Activity Duration?
The precision in duration estimation of an activity depends on the quantum of work involved in it. The higher the duration of an activity, the lesser is the precision in duration estimation. DCMA recommends the activity to have a lower duration than 44 days. If unavoidable, such activities should not be more than 5% of the total activities.
To accurately determine the time duration for an activity, the following data is required-
- Complete Scope and inter-dependencies of the activities.
- Available resources.
- Priority of the job and criticality of the activity.
- Working space availability.
- Risks involved in the activity.
- Time estimates from previous projects of similar nature.
The choice of the technique varies with the project, industry and organisation. It is the key responsibility of the project manager to determine the correct duration estimation technique.
The top six project time estimate tips & tricks are listed below.
1. Bottom to Top Estimation
The bottom to top estimation method comprises of duration estimate of smaller activities and summarising them to get the complete project duration. The method is backed up by the data available from previous projects of similar nature.
The method is highly accurate as each activity is drilled down to the lowest level and is discrete in nature. Bottom to top approach works well with projects that have clearly defined activities and clear scope such as construction projects. It is a quick method to assess the resources required for the project.
2. Top to Bottom Estimation
The project is divided into work packages. The packages are assigned with the duration estimate. These packages are further divided into smaller activities when sufficient information gets available.
Top to bottom method is suitable for initial planning and schedule. The planner can suitably divide the work package and determines the duration using historical data.
3. Expert Judgement
The expert judgement technique can be used in conjugation with the top to bottom and bottom to top technique. An expert’s opinion on duration estimation is bound by his/her experience in similar nature projects, understanding of the project’s complexity and analytical ability.
A planner should also consult the team members while deciding the duration of an activity. The team is often comprised of people having experience of similar projects. Thus, their input in duration estimation is valuable.
The method is useful in complex projects and estimation is to be done beyond the numerical data.
The drawback of this method lies in the fact that it is opinion-based. The opinion for duration estimate may change from person to person and hence not too reliable.
4. Analogous Estimation
The analogous estimation technique uses the data from previous projects of similar nature. This method helps in estimating when and when not. The positive and negative feedback both help in determining the duration of an activity.
It is the fastest method to estimate the time required for a job to complete. However, similar projects may have different constraints that affect the project completion. Thus, the analogous estimation may yield wrong data if the constraints of new and old projects are not well understood.
5. Parametric Estimation
Parametric estimation is done by dividing the project scope into work packages of similar nature and identifying the time taken by a single work package.
The duration estimate of any work package is determined either by historical data or expert’s opinion. Thus, the accuracy of this method depends on the quality of data and its’ interpretation.
The method is useful for projects that have repetitive activities such as pipeline construction, power transmission line construction etc.
6. Three-Point Estimation
The three-point estimation is widely used in research projects and product development. The technique is highly suitable for projects that have high uncertainty and the duration estimate is difficult.
The three-point technique uses three-time estimates to determine the likely time required to complete the activity. The three estimates are-
- Optimistic Time (to)- The time duration in which the activity can be completed if all the ideal conditions are met.
- Most Probable Time (tm)- The time required for an activity to be completed in normal working conditions.
- Pessimistic Time (tp)- The pessimistic time is the minimum duration of activity if all the adverse conditions prevail.
The likely time to complete the activity is calculated as follows-
The technique selection depends on the scope, details available, historical data availability, complexity of the project, bottlenecks and constraints.
For the detailed scope project, bottom to top technique is best suited. If the detailed scope of work is not defined, top to bottom technique provides a reasonable time estimate.
The research projects and new projects that have no historical data available, expert judgement is crucial. If the experts’ opinion widely differs, a common ground should be established and a three-point estimate technique should be used.
For projects of the same nature and similar scope, the analogous technique works well. It saves a lot of effort. The projects having repeated work packages should use the parametric technique.
Learn to create a construction project schedule with an example here.
Read my experiences of waterfall project management here.
Read about the schedule and project management here.