Nelson Bustamante: How to Estimate Duration of Activities in Construction

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In the world of project management, scheduling procedures are fundamental to the success of any endeavor. Central to these procedures is the estimation of activity durations, a process crucial for developing a comprehensive project schedule. This article delves into the methods and factors involved in estimating activity durations, highlighting the iterative nature of the process and the significance of predecessor relationships among tasks.

Estimating Activity Duration

The accuracy of a project schedule hinges upon the precision with which the durations of various project activities are determined. Whether it’s a project phase or a product phase, estimating activity duration is an iterative process that evolves as work progresses and new activities are identified. At the core of this estimation is the utilization of probability distributions, which provide insights into the likelihood of specific activity durations.

Analogous Estimating

One prominent technique for estimating activity duration is analogous estimating. This method relies on historical data and information from past projects that share similarities with the current endeavor. While not as precise as other techniques, such as parametric estimation, analogous estimating offers a cost-effective and time-efficient approach to schedule development. However, it’s essential to acknowledge that the unique characteristics of each project can influence the ease or difficulty of completing specific activities.

For instance, challenges like limited access to resources or specialized equipment may impact productivity rates, necessitating adjustments to the estimated durations. To calculate activity durations using analogous estimating, factors such as required resources, productivity rates, and the number of assigned crews are considered.


Aij: required formwork area to assemble, typically determined from a detailed examination of the final facility design.

Pij: average productivity
of a standard crew in this task (measured in square yards per hour), commercial
services can provide average productivity figures for many standard activities
of this sort. Historical records in a firm can also provide data for estimation
of productivities.

Nij: number of crews
assigned to the task which are decided by the planner

A formula such as Equation 1 can be used for nearly all construction activities. Sometimes, adjustments based on engineering judgment are made to the calculated duration from Equation 1 for this reason.

Factors Influencing Productivity Rate (Pij)

Inaccurate estimation of project activity durations often stems from uncertainties and changes in productivity rates. Understanding the factors that influence productivity is paramount for mitigating such inaccuracies:

1. Systematic Factors

1.1 Effect of Learning on Productivity Rate

As individuals or teams become more familiar with tasks over time, their productivity tends to improve. This phenomenon, known as the learning curve, manifests in increased efficiency and reduced completion times for repetitive activities.

    Illustration of Productivity Changes Due to Learning
    Fig. 1: Illustration of Productivity Changes Due to Learning

    1.2 Effect of Weather on Productivity Rates

    1. Weather conditions can exert a significant influence on productivity, particularly in outdoor projects. Incorporating adjustments for inclement weather and leveraging meteorological records enable more accurate estimations of activity durations.

    2. Random Factors

    2.1 Skill Level of Personnel:

    1. The proficiency of personnel assigned to a project can vary widely, impacting productivity rates. While average values may be used in the absence of specific knowledge, variations in skill levels introduce uncertainty into duration estimates.

    2.2 Weather Variability

    Random weather events, such as rainstorms, can disrupt project activities and introduce unpredictability into schedules. Assessing the likelihood and impact of such events is essential for effective schedule management.

    Alternative Estimation Techniques

    Beyond analogous estimating, project managers can employ alternative techniques such as parametric estimation, expert judgment, and work breakdown structure analysis. Each method offers unique advantages and is suited to different project scenarios, contributing to the versatility and adaptability of project scheduling practices.

    In summary, effective project scheduling hinges on the meticulous estimation of activity durations, informed by historical data, environmental factors, and the expertise of project stakeholders. By understanding and addressing the complexities inherent in duration estimation, project managers can cultivate schedules that facilitate successful project outcomes.

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