One of the keys to a good schedule is accurately estimating activity durations. A common mistake with new project managers and schedulers is to try and estimate the activity duration for each task, often thinking in terms of how many linear days it might take to complete, this is known as ‘Guessing.’
When estimating, you need to consider that every activity has three attributes:
- Duration – the number of days, weeks, or months the activity will run
- Effort – the total hours required to complete the work
- Resources – quantity required; availability, skill level, etc.
In this article, we will look at some of the standard techniques and tools used for getting the best estimate of activity duration possible. We will also discuss how using Primavera P6 can help with estimating activity durations.
There is a sequence of steps that helps with estimating activity durations as you put together a project schedule, as shown below in Figure 1.
The first step in the process is to define the activity list. At this point, we’re not concerned with their durations; we simply want to make sure we capture all the likely activities required to complete the project.
A good Work Breakdown Structure (WBS) is helpful because it already outlines the major deliverables for the project. All we need to do is make sure we assign the appropriate activities to the correct sections of the WBS.
Next, we need to get these activities into a logical sequence. To do this we define relationships between the activities to show their dependence upon preceding work completed. For example, we cannot install a roof until all the walls are erected.
Once we have a completed and sequenced list of activities, we can start to consider the best techniques for estimated their durations.
Estimate Activity Duration Inputs
Some examples of inputs that can be used in the process of activity duration estimation are:
- Activity List – a comprehensive list of all schedule activities required for the project. The activity list includes the activity identifier and the scope of work description that should be adequate for a team member to understand the work required.
- Activity Attributes – these include the activity ID, WBS ID, name or description, activity codes, relationships to other activities, resource requirements, date constraints, activity type (level of effort, discrete effort, etc.), and assumptions.
- Milestone List – a list of the significant points or events within the project such as contractual mandatory dates or key deliverables.
- Project Scope Statement – this contains the project scope description that includes product characteristics that may affect activity sequencing.
Tools and Techniques for Estimating Activity Durations
At the estimating activity durations stage of schedule development, there are several tools and techniques you can employ to get the best estimates for your activities:
1. Expert Judgment – this method relies upon the experience of individual subject specialists and the combination of historical information. These individuals can recommend maximum activity durations based on their experience with similar projects. This is the most common source of information used, about the development of activity durations.
2. Analogous Estimating – this method uses such parameters as duration, budget, size, weight, and complexity based upon a similar project. Historical data based upon actual durations of activities completed in previous projects can be used as a basis for duration estimate, taking into account differences in size and complexity. This method is generally less costly and time-consuming than other methods, however, it is often less accurate. It works best when the work is similar in fact and not just the appearance of prior work.
3. Parametric Estimating – this method uses statistical relationships between historical data and other variables to calculate an estimate for activity parameters. For example, if one Tile Setter can lay down 100 square feet of tile per day based upon previous actual data, you could estimate that a 500 square foot floor will take one Tile Setter 5 days to complete. Or two Tiles Setters 2.5 days, or five Tile Setters 1 day to complete.
4. Three-Point Estimates – this method originated with the Program Evaluation and Review Technique (PERT). The basic principle is to take three estimates, namely Most Likely, Optimistic and Pessimistic, and derive an approximate range for an activity’s duration.
- Most Likely (tm) – the estimated duration of the activity is based upon the likely resources assigned, their productivity, availability, dependency on other protagonists, and likely interruptions.
- Optimistic (tO) – duration is based upon analysis of the best-case scenario.
- Pessimistic (tP) – duration is based upon analysis of the worst-case scenario.
- The Expected (tE) value is calculated by PERT using the following equation, shown in Figure 2, to obtain a weighted average of these three estimates.
An example of a Three-Point Estimate
- Most likely = 20 days
- Optimistic = 10 days
- Pessimistic = 25 days
Following the calculation shown in Figure 3, Estimated = 19.17 days.
Another example of an important aspect to consider when estimating activity durations, are environmental factors that can directly affect how quickly work can proceed.
For Example:
A roofing contractor knows that he can replace an entire shingle roof on a 1500 square foot ranch with a team of 5 roofers in one day. However, that is a single-story building with a 25-degree pitch roof with only one layer of existing shingles to remove. The roofing contractor has many variables to consider beyond simple square footage calculations when estimating the duration of each activity for the project. Items that will affect the estimate are as follows:
- Area of the roof – how many Squares (10’ x 10’ area) need to be covered?
- Pitch of the roof – steeper pitches are more difficult to work on and require additional equipment such as roof jacks and ridge ladders.
- Roofing materials – shingles, concrete tiles, metal, slate: all of these materials require different skills and equipment to install and thus can be done faster or slower.
- Weather – a risk factor in roofing at any time of the year, in almost any area. During wet seasons more work is required to protect the building while the roof is being stripped and is exposed to the elements.
- Unknown Conditions – how good is the boarding below the old roof? Will boards have to be replaced when unexpected problems are uncovered?
Many factors will influence the duration of the roofing activities. Skill, historical data, and experience will play a big role in the accurate estimating of projects for this contractor.
Using Primavera P6 to Help with Estimating Activity Durations
Primavera P6 can help with estimating activity durations. Activity durations can be calculated in Primavera P6 using the Duration Type attribute, as shown in Figure 4.
Earlier we looked at the attributes that determine an activity’s duration. These were:
- Duration – the number of days, weeks, or months the activity will run for.
- Effort – Total hours required to complete the work.
- Resources – quantity required; their availability and their skill level.
Primavera P6 accommodates these attributes by providing us with the same controls. You can map these attributes to a P6 activity in the following way:
- Duration = Duration
- Effort = Units (the total number of units, typically hours, required to complete the work.)
- Resources = Units/Time (a resource assignment attribute that typically represents the hours per day a resource is available to work on a particular activity).
Duration Type
Duration Type is used to determine how Primavera P6 will maintain the balance between Duration, Units/Time, and Units when resources are assigned to an activity, as shown in Figure 5.
The relationship between these three elements must always reconcile. If one is changed, at least one of the others must be recalculated. If you reduce the duration of a task, then you have to either reduce the total number of hours it was supposed to consume (cut scope) or increase the number of people to achieve the same outcome.
Because not all activities can be reduced in duration by adding people to the task, the correct duration type should be applied to make sure P6 reconciles future changes correctly.
The Duration Type column offers four values:
- Fixed Duration and Units
- Fixed Duration and Units/Time
- Fixed Units
- Fixed Units/Time
These values represent four ways in which P6 can handle the relationship between the duration of the activity, the units of work assigned (hours), and the units/time (effort) of that assignment.
For Example, if we have a 10-day duration activity, with one person working on it full time;
Duration = 10
Units (hours) = 80
Units/Time (effort) = 8/d
If we change one of the values, what other values will P6 recalculate to reconcile the original assignment levels?
By setting the correct Duration Type, P6 will calculate in a predictable way whatever you have allowed it to.
Below are examples of the four Duration Types. Using our 10 Days duration activity with one full-time resource we review how changes affect the Units/Time.
Fixed Duration and Units
If either the Duration or the Units value is changed, Primavera P6 will recalculate the Units/Time (effort) for the activity.
For example, if the duration is reduced to 5 Days, P6 will recalculate the Units/Time from 8/d to 16/d. It recalculated Units/Time because the other two attributes are fixed.
- Duration = 5 Days
- Units = 80
- Units/Time – 16/d
One person working 16-hour days to complete a ten-day activity in five days.
If the Units value is changed from 80 to 40 on the 10 Days duration activity, P6 will recalculate the Units/Time to 4/d.
- Duration = 10 Days
- Units = 40
- Units/Time = 4/d
One person working half the time to complete 40 hours of work in ten days.
Fixed Duration and Units/Time
With this option, Primavera P6 can only recalculate the Units when one of the other two values is changed. Therefore, if the duration for the task is changed, P6 will update the value of the Units and if the Units/Time value is changed, P6 will update the Units value to reconcile to the original assignment.
For example, if the duration is reduced to 5 Days, P6 will recalculate the Units from 80 to 40. P6 recalculated Units because the other two attributes are fixed.
- Duration = 5 Days
- Units = 40
- Units/Time – 8/d
One person working 8-hour days to complete a ten-day activity in five days.
Fixed Units
With this option, P6 can recalculate either the Units/Time or Duration.
Using our ten-day activity example again: if we reduce the Units value from 80 to 40, P6 will recalculate the Duration to 5 days.
- Duration = 5 Days
- Units = 40
- Units/Time – 8/d
One person working 8-hour days to complete 40 hours of work for over 10 days.
If we reduce the duration from 10 days to 5, P6 will recalculate the Units/Time from 8/d to 16/d.
- Duration = 5 Days
- Units = 80
- Units/Time – 16/d
One person working 16-hour days to complete 80 hours of work in 5 days.
Fixed Units/Time
With Fixed Units/Time the Duration and Units can be recalculated but Units/Time cannot.
For example, if the Duration is reduced from 10 days to 5 days, the Units will be reduced from 80 to 40, but the Units/Time remains at 8/d.
- Duration = 5 Days
- Units = 40
- Units/Time – 8/d
One person working 8-hour days to complete 40 hours in 5 days.
Again, if the Units are reduced from 80 to 40, the duration will be recalculated from 10 days to 5 days.
- Duration = 5 Days
- Units = 40
- Units/Time – 8/d
One person working 8-hour days to complete 40 units in 5 days.
The initial duration is set when the activity is created and the initial Units and Units/Time values are set when the first resource is assigned. Subsequently, assignments on the same activity will affect these initial settings depending on the Duration Type used.
Calculating Activity Duration using Duration Type
Using the Fixed Units duration type, Primavera P6 will calculate the duration of an activity. We have estimated that an activity is going to require a particular number of engineering hours to complete and we want P6 to tell us the duration of the activity-based upon the number of engineers we can assign to it during a particular period.
Activity A1000 – Engineering Design is estimated to take 300 hours of engineering time to complete. We want to know the duration of the activity if we have just two Engineers to do the work.
First, we set the Duration Type of the Activity to Fixed Units in the General tab of the Details area, shown in Figure 6.
The activity is currently 20 days in duration and has no resource assigned to it. Next, we assign the first Engineer to the activity using the Resources tab, as shown in Figure 7.
With one Engineer on the activity, P6 has calculated an overall Budgeted Unit cost of 160. However, we need 300 units (hours) to complete the work. So we need to adjust it manually. Remember this is a Fixed Units Activity Duration Type, so P6 isn’t going to recalculate the Units value.
The value of 300 is entered into the Budgeted Units field, as shown in Figure 8.
Primavera P6 has recalculated the duration of the activity to 38 days based upon the total number of hours required to complete the work and a Budgeted Units/Time value of 8 hours per day. In other words, we have calculated the Duration based upon just one Engineer working on the activity, as shown in Figure 9.
However, we have two Engineers available during this period, so we can increase the hours per day to reflect this and see what duration P6 calculates for us. With two Engineers working on this full time, we will be able to complete the activity in 19 days, shown in Figure 10.
Notice how P6 didn’t recalculate the Budgeted Units value. Instead, it recalculated the duration because we have told it that the Budgeted Units are fixed – leave that value alone wherever possible.
What if we need to get it done even faster? Let’s say that we can get another Engineer to work part-time on the activity, say 2 hours per day.
We’ll increase the Budgeted Units/Time value from 16h/d to 18h/d to see if this will help us. As you can see, this has reduced the Duration by a further two days bringing the estimated duration to 17 days total, shown in Figure 11.
The real power of this is behind the scenes. Both the activity and the resource are working on a calendar that reflects the working days for the schedule and the resource. If the resource was working to a different calendar, then the Activity Type can be changed to Resource Dependent thus driving its duration based upon the resource calendar, not the activity calendar.