Construction Management Engineering

Estimating Activity Duration In Project Management

Estimating the duration of the activity is the process of estimating the number of work periods required to complete individual activities with estimated resources. The main advantage of this method is that it gives the quantity of time each activity will take to finish, which is a significant contribution to the process of developing the schedule.

Analogous Estimating

The analogous estimating is when you make an analogy to the same or (similar) activity that has been performed before.  This is the best source of info because actual work completed is reliable, even if it needs adjustments.

Here is the potential adjustments checklist :

  • The activity is smaller or bigger than the last one.
    In this situation, scale down or up to the length by an appropriate quantity.  The log house, for instance, has slightly more square ft, or the sidewalk is longer, or more windows have been added.  This may sound evident, but it’s never the large, apparent things that cause project managers issues.  You have to create sure that for every little thing you include enough contingencies.
  • The activity needs special services or products that the previous one didn’t.
    In this case, add and proceed as usual in the appropriate products and services. Usually, by missing a specialty item, there is a strong potential for project overruns. That is, roof trusses that are slightly larger may require the next larger crane size.
  • If the activity for an end product is slightly different.
    Determine the material or/and workmanship difference here and determine the resource, price and time difference.  For example, from the plans, you could calculate that the log home requires 20% more timber than the previous one.  Then you could estimate that it will take 20 percent longer for the Wood Work task.

Analogous estimating top issues is that it’s only as reliable as actual data the estimate is being taken from.  Often you need to have been the project manager of the previous project, or at least close enough to it to know what happened & how it went. i.e, the crane broke down and spent four days on site instead of two, you would not necessarily have to take that into account in the new estimate.

Also, work productivity is an important consideration.  Any experienced laborer has higher productivity than a less experienced one. For knowledge workers such as engineers, adding more of them sometimes slows the production & decision making process down instead of speeding it up. Therefore, switching funds from project to project may not lead to contrast between apples and apples.

Parametric Estimating

A second common method for estimating the duration of activity requires the work to be drilled down to a unit cost, e.g. per square foot of log home. Below are a few examples

  • Log home’s Total construction time per square foot
  • Pouring time per cubic yard of concrete.
  • Installation time of carpet per square foot.

In the construction industry, almost everything is performed this way, from engineering to the building materials. The parametric estimate of the project would serve as an excellent guide for the business as a whole (determine the duration or cost per sq ft of home).

If you have many similar projects in your company, you need to ensure that these parametric estimates are available and up to date.

Sometimes a parametric value involves a fixed element, such as doors and windows that require a set time for an installer to arrive at the site and so on. In this case, it is possible to separate the fixed items and add them to the parametric estimate.

Three-point Estimating

In this method, the estimator determines three numbers:

  1. Optimistic
  2. Most Likely
  3. Pessimistic

Most likely is the average of what the duration of the activity would be if it were performed multiple times, that is, the normal estimate using other methods. The other two are the reduced & upper boundaries you are unlikely to exceed.

To do this numerically, you would assign the upper and lower boundary confidence intervals (optimistic and pessimistic values), such as 90%.  Several textbooks use 99% or some other value, the methodology is different.  But if you have 10 prior projects to consult with and can determine what value 1 out of 10 projects has surpassed, you can do so.

Generally, one of the two distributions is used.  The triangular distribution is simply an average of the three estimates & the beta distribution is a bit tighter.

Triangular (average) distribution Beta distribution
te = (a + m + b) / 3 te = (a + 4m + b) / 6

Where:

  • te = Expected duration
  • a = Optimistic
  • m = Most likely
  • b = Pessimistic

The triangular distribution will skew the results away from the Most Likely value to the value.  The beta distribution is tighter, that is, the outcome is nearer to the value of Most Likely.  I don’t think this involves more rocket science than that.  You just have to decide how close you want to be to the Most Likely value.

Watch Duration Estimation Types, Inputs, Methods, Parametric Estimation

See Also

S-CURVE EXCEL TEMPLATE
WHO OWNS THE FLOAT?
UNDERSTANDING PRIMAVERA P6 DATE INFORMATION

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