What methods are used to estimate task duration?

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What is Activity Duration?

Activity duration is the time between the start and finish of a schedule activity. For example, if we consider walking to your nearest McDonalds for a coffee an activity, it depends on how fast we walk. Lets say it is approximately 500 meters from your house, if you walk at a comfortable pace, you will take around 10 mins to reach the place.

So, the duration of the activity of walking to your nearest McDonalds is 10 mins. Once you know this number, you can prioritize or plan accordingly to meet your needs.

Tools and Techniques for Activity Duration Estimating

The project schedule depends upon the activity duration estimates. The duration estimates of activities on the critical path will determine the finish date of a project for a given start date. However, there might be many uncertainties involved in the estimate. For example, two programmers, due to the differences in their experience, will take different amounts of time to write the same program. The good news is that there are a number of tools and techniques that you can use in activity duration estimating.

Analogous estimating - Analogous estimating techniques estimate the duration of an activity based on the duration of a similar activity in a previous project. The accuracy of the estimate depends upon how similar the activities are and whether the team member who will perform the activity has the same level of expertise and experience as the team member from the previous project. This technique is useful when there is not enough detail information about the project.
Parametric estimating - This is a quantitative technique used to calculate the activity duration when the productivity rate of the resource performing the activity is available. You use a formula such as the following one to calculate the duration:

Activity duration = Units of work in the activity / Productivity rate of the resources

For example, if you know that a team assigned to the activity of laying a road for 40 miles can lay two miles of road in one day, the duration calculation can be performed as follows:

Activity duration = 40 miles / (2 miles/day) = 20 days

Three-point estimating - This method addresses the issue of uncertainty in estimating the activity duration. The uncertainty in the duration estimate can be calculated by making a three-point estimate in which each point corresponds to one of the following estimate types:
Most likely scenario - The activity duration is calculated in most practical terms by factoring in resources likely to be assigned, realistic expectations of the resources, dependencies, and interruptions.
Optimistic scenario - This is the best-case version of the situation described in the most likely scenario.
Pessimistic scenario - This is the worst-case version of the situation described in the most likely scenario.

The spread of these three estimates determines the uncertainty. The resultant duration is calculated by taking the average of the three estimates. For example, if the duration for an activity is estimated to be 20 days for the most likely scenario, 15 days for the optimistic scenario, and 25 days for the pessimistic scenario, then the average duration is 20 days, and the uncertainty is ± 5 days, which can be expressed as:

Duration = 20 ± 5 days

It's equivalent to saying that the activity duration is 20 days, give or take five days.
However, the most likely scenario may be given more weight than the other two scenarios. Therefore, the expected duration can be calculated by using the following formula:

ED = ((N*MD) + OD + PD)/(N+2)


ED = Expected Duration
MD = Most likely Duration
OD = Optimistic Duration
PD = Pessimistic Duration
N = Weightage for the Most likely Duration

Reserve analysis - Reserve analysis is used to incorporate a time cushion into your schedule; this cushion is called a contingency reserve, a time reserve, or a time buffer. The whole idea is to accommodate the possibility of schedule risks. One method of calculating the contingency reserve is to take a percentage of the original activity duration estimate as the contingency reserve. It can also be estimated by using quantitative analysis methods. Later, when more information about the project becomes available, the contingency reserve can be reduced or eliminated. Usually, while estimating for large projects, managers would like to keep a buffer of 5% or so of the total estimate for the project to account for project schedule risks, like delays in procuring hardware or unexpected human resource risks (Ex: attrition) etc

Expert judgment - Expert judgment can be used to estimate the whole duration of an activity when not enough information is available. It can also be used to estimate some parameters to be used in other methods. For example, what percentage of the original activity duration estimate should be used as a contingency reserve, and in comparing an activity to a similar activity in a previous project during analogous estimation.

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