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"Minimum, eighty percent," comes the answer, "preferably ninety."

No arguments.

"Mark, now we understand why you put your time estimate way to the right of the probability distribution. It is somewhere around eighty or ninety percent."

"Sure."

Addressing the class, I declare, "The difference between the median of the probability distribution and the actual estimate is the safety we put in." And I pause to give them time to think it through.

I turn back to Mark. "So, in your time estimates, you do include safety to protect yourself against uncertainty, or in your words, Murphy?"

"I guess so."

"When we compare the time indicated by the median to the time you indicated as a reasonable estimate, it doesn't look like the safety you added is in the range of twenty percent." "Closer to two hundred percent," he admits.

"Look at the graph," I urge them. "Do you understand that the time estimate that gives us a fifty percent chance is much shorter than the time estimate that provides an eighty percent chance of completing a step before the estimated time? And don't forget, the bigger the uncertainty, the bigger the difference."

"So, two hundred percent safety and more is the norm, not the exception," Ruth says thoughtfully.

"Except for some over-zealous engineers, everybody makes estimates at the range of over eighty percent chance," I say.

"Which means that for each and every step of the project we factor in a lot of safety. Are you starting to realize," I ask the class, "the extent to which we do insert safety into our projects?"

They all nod, trying to digest it. Mark returns to his seat, still looking over his shoulder at the probability distribution displayed on the board. I wait until I'm convinced that Mark and the two unfortunate students he tripped over are okay, and start my summary.

"We concluded that the uncertainty existing in every project is the underlying main cause for most problems. Now we see that people are not blind to it and they do add a lot of safety in their planning. Do you agree that we must investigate, more deeply, this issue of safety?"

Full consensus.

"Good," I say. "So here is your homework assignment. Go back to the project you examined and pick, arbitrarily, at least three different steps from that project. For each of the steps you selected find out how the time estimate was arrived at. Don't just ask the project leader. Find out who gave him or her the estimate and interview the source."

Ted raises his hand in apparent discomfort.

"What's the matter, Ted?"

"It's not so simple."

"Why?"

Ted is still looking for words when Charlie answers, "Because, many times a step appearing on the chart of the project leader actually represents many tasks done by many different people."

"Several people are involved in generating the time estimate for even one step in the project," Brian elaborates.

"So you'll have to do some digging," I cynically say. "Some digging," Ted echoes. "What an understatement. It's a lot of work."

"And most of it is not documented anywhere," Brian continues to express his concerns. "I wonder if people will remember how they derived the original estimates."

"You'll have to do your best," I reply. "Remember, we just concluded that it's vital for us to gain a better understanding of this issue of the safety embedded in the planning of a project. I can tell you that very little exists in the literature about this topic. If we want to make any headway, we'll have to dig up the data ourselves. There is no other choice."

"It's a lot of work," Ted is vocal. "We can't do it for next session."

I try to argue, but there is little you can do against a unified class. It's a problem. I'll have to deviate from the sequence I had planned to follow. On second thought, it won't be too disruptive. I can devote the next session to the subject of PERT and critical path. We compromise, and agree that they will submit the assignment the session after next. At least they promise to do an in-depth