Thursday, January 17, 2008

Project Tools: Quantitatively-based Durations

If you had five lawns to mow and you knew it took you about an hour to mow one lawn, then how long would it take you to mow all five? Pretty simple, right? Well, that's how easy it is to use quantitatively-based durations to estimate project activity durations.

Quantitatively-based durations are often the simplest types of estimates to make, provided you have accurate activity information with which to work. The two types of activity information required to calculate quantitatively-based durations are quantities of work and productivity unit rates.
  • Quantities of work to be performed refers to work amounts. Some examples of quantities of work to be performed for each activity are number of graphics, meters of cable, and tons of sand.
  • Productivity unit rates refers to the length of time required to complete a fixed amount of work. Examples of productivity unit rates include hours per graphic, meters of cable per hour, and tons of sand per day.
Consider the example of a contractor who is building a new overpass as part of a highway project. Based on the size of the overpass, the project manager knows that she will require about 40 tons of concrete to complete the project. Since she will be working with a crew that she has worked with before, she knows it will take around five days to pour each ton of concrete.
In the example above, 40 tons of concrete represents the quantity of work, while the productivity unit rate for the project is five days per ton of concrete.

Understanding quantities of work to be performed for each activity and productivity unit rates provides useful information to project managers. This information is useful because it provides the data required to perform quantitatively-based duration calculations.

Quantitatively-based durations are calculated by multiplying the quantities of work to be performed for each activity by their productivity unit rates.

Remember, quantitatively-based durations can help you perform activity duration estimates, which can help you avoid surprises that could throw off your project schedule.

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