## Estimating Construction Activity Durations

Feb 25, 2014

A number of specific methods can be used to arrive at reasonably accurate estimates of the time required to perform a given activity or piece of work on a job. Before getting into the details of these methods, however, some general rules should be noted. These rules can help to arrive at realistic times and avoid serious errors, and should be observed by anyone developing a construction schedule.

Assume Each Activity Will Be Done Normally

First, the time estimates should initially assume a normal or ideal set of working conditions. For most activities, there is a most efficient rate of production which results in the lowest possible unit cost. Because all jobs vary somewhat, this rate of production is not a precise number. Instead, it represents a range of production rates, which, experience has shown, result in the lowest unit cost. If the production rate is significantly higher or lower, the unit cost changes accordingly. In the best of all possible worlds, a construction manager would expect all the activities on a job to be carried out at a rate close to the ideal. In the real world, the ideal rate is actually possible for most activities. However, those that do not proceed on schedule may be critical to the job, or hold up the progress of other activities in some way. The best procedure, therefore, is to plan all activities initially in terms of ideal time, and then change only those that must be changed for valid reasons, such as overall time.

Evaluate Each Activity Independently

In addition to initially assuming an ideal time for each activity, the scheduler should compute individual activity times as if no other work existed. Clearly, this is not realistic in the long run, but practice has shown that much of the work of a project does in fact proceed without being affected very much by other work. It is also true that if a person drawing up a schedule tries at the start to consider every constraint affecting an activity, the number of variables may rapidly become overwhelming. Further, many of the factors that will affect a given activity cannot be known until the overall project time has been determined. Thus, it is better to plan each activity independently, and then account for constraints as necessary and as they become known in the scheduling process.

Use Consistent Time Units

Days are by far the most common unit of time measure in the construction industry, although hours or weeks can be used appropriately in many circumstances. Work days are converted to calendar days. This task can be easily computerized, which is typically done.

Regardless of the time unit used, it is important to be consistent in order to prevent confusion and misunderstanding over the scheduled times for various parts of the job. Calculated times for work activities are usually in work days; the quoted times for delivery of materials are often in calendar days. The person preparing the schedule must be certain that one is converted, if necessary, to be consistent with the other.

Keep Good Records as the Schedule Is Developed

It is often helpful during the schedule development process to be able to refer to previous assumptions, calculations, and “trial balloons.” For example, in trying to decide how much to speed up various activities, it is helpful if one knows what is the assumed normal rate. With this information, it is possible to gauge the effect of one activity’s acceleration on the various other activities, so as to decide on the most efficient mix. Toward this end, most good schedulers maintain analysis and record sheets on each activity as the schedule develops.

Typically, any given activity may have several sheets worked out over the course of the job. Dating and keeping each subsequent sheet in order is important. The sheets might be kept in a loose-leaf notebook, catalogued by a classification system, such as the CSI MasterFormat or by phase within the project. An organized approach makes referring back to previous data much easier.

Excerpted from Project Scheduling and Management for Construction, available through RSMeans

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