Time Buffers Account for Variability

As discussed in the previous article (1 of 5), an everyday task such as driving is a relatable activity that can help us think through the issue of production scheduling. The goal of our thought exercise is to apply the concepts to our manufacturing scheduling practices and improve our operations.

To begin, our stated goal is to arrive at work no later than 8:00 a.m. To plan our departure time, we’ll break apart our imaginary drive into “ideal” segments that are based on distance and speed limit. The first segment is 5 miles with a 30 MPH speed limit, the second segment is 10 miles with a 40 MPH speed limit, and so on. Using this method, we estimate that our 31-mile drive will take 43 minutes to complete. In other words, if we’re able to travel at the speed limit without any stops or slowdowns for each of the segments, it will take us 43 minutes to get to work.

Now, we need to add a time buffer to account for variability. The time buffer will help budget for the portions of our trip when we will be traveling at less than the speed limit or not moving at all. The time buffer we choose will be influenced by the penalty, or the financial loss we are prepared to accept in case we arrive late to work. Think of the penalty as a threat to our well-being; as such, it is something to be avoided.

As we think about choosing the right time buffer, consider the following:

  • “On time” means not late.
  • Due to variability, we should not expect to arrive to work at exactly 8:00 a.m very often. We will almost always be early, and we will be late very infrequently.
  • It is possible for us to leave late and still arrive on time, but we shouldn’t make a habit of it. Variability will get us!

Now, Let’s Plan Our Drive

With the above in mind, we can plan for our drive. If we need to arrive at work no later than 8:00 a.m., our possible departure scenarios might include:

  • Leave at 7:17 a.m. – allots 43 minutes to arrive exactly at 8:00 a.m. This departure time assumes that there will be no delays that cannot be made up by exceeding the speed limit. In other words, this option has a time buffer of zero minutes, meaning the threat of late arrival is very high.
  • Leave at 7:00 a.m. – allots 60 total minutes including 17 minutes for unplanned delays. It’s a nice round number that’s easy for us to recall in the morning (with or without coffee!). The threat level is moderate because we will regularly arrive on time; although it is possible that on some occasions, we may not.
  • Leave at 6:30 a.m. – allots 90 total minutes including 47 minutes for unplanned delays. We hope to minimize the threat level of losing our job even though it is likely to result in arriving frequently early to work. For example, we may have security, second shift, or other responsibilities that require us to report on time. If buffer time remains upon our arrival, we may reward ourselves with a coffee or snack before our work begins.

In the next article (3 of 5) in this series, we’ll discuss the implications of our thought exercise for manufacturers.

What buffer size are you comfortable with? I.e., where is your balance point between arriving “too early” and the risk of being later? How much variability do you have to contend with each day during your commute?