A Simple Formula to Keep Projects on Time


For many years, science has proven that we can only do one thing at a time. More specifically, we can attend to only one cognitive task and process only one mental activity at a time: we can either talk or read but not do both at the same time. We can only have one thought at a time, and the more we force ourselves to switch from one thing to another, the more we tax our mental faculties.

In 2001, Joshua Rubinstein, Ph.D., Jeffrey Evans, Ph.D., and David Meyer, Ph.D., conducted and published four experiments in which young adults switched between different tasks, such as solving math problems or classifying geometric objects. The findings for all tasks revealed:

  • The participants lost time when they had to switch from one task to another.
  • As tasks became more complex, the participants performing them lost more time.
  • As a result, people took significantly longer to switch between increasingly complex tasks.
  • Time costs were greater when the participants switched to tasks that were relatively unfamiliar.

It is important to pull ourselves away from work now and then; in fact, I teach my students a rhythm of 25 minutes to task, followed by a five-minute break, followed by another 25 minutes (repeat until the task is complete). Breaks are one thing, but distractions are another. Breaks are short, focused and deliberate. Distractions catch us off guard and derail our task entirely.

Meyer has said that even brief mental blocks created by shifting between tasks can cost as much as 40 percent of someone’s productive time. It seems unlikely that we will be able to refocus company culture to accept the virtues of scheduling and completing one task before starting another.

If this behavior is inevitable and unpredictable, what can we do to keep our projects on time? Perhaps the answer is a single point estimate using the resources’ availability and productivity.

Single Point Resource Availability and Productivity Technique

Estimating time in projects is typically done using a single point estimate derived from experience and best guess. At best, a single point estimate gives us a 50 percent probability of success. We can increase those odds a few percentages by accounting for both the availability of a resource and their average productivity, which we have learned through studies is between 72 to 74 percent. I will use a constant of 70 percent for simplicity.

In Equation 1 that follows, d represents duration, e represents the amount of effort needed to complete the task, a represents the resource’s availability and p represents the average productivity of a typical knowledge worker. Equation 2 solves for d.

d = \frac{e}{a}/p
Equation 1. Single Point Estimate Using Availability and Productivity


d = \frac{10h}{50\%}/70\%

d = 20h/70\% = 26\:hours
Equation 2. Single Point Estimate Using Availability and Productivity Example

Strengths of this technique

This technique is helpful when the hours given come with high confidence and the work is fairly routine and easy to calculate.

Weaknesses of this technique

We all know how difficult it is for most people to articulate, with any predictability, the accuracy of their timelines. Multitasking, or task-switching, is rampant in our fast-paced culture. Since this work ethic is not likely to change, we must use probabilistic math.

There are many ways to estimate time and some are more accurate than others. All come with uncertainty. We have a tendency to base our project estimates on the best guess, which makes it difficult to plan for the inherent randomness in these estimates. At best, a single point quote would only give us a 50 percent likelihood of success. We may increase those odds by looking at a best and worst estimate, plus the resource’s general availability and productivity.

You can learn about other project time estimation techniques in my white paper, A Toolkit for Project Time Estimation.

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