Sometimes it will be ready early, and will have to wait for the next activity. Sometimes it will be ready late, when the downstream process is no longer available. Thus, you cannot use average task durations to plan engineering schedules whenever there is uncertainty in these durations. Fortunately, this is a quantitative problem that engineers have worked on for the last 80 years. It is analogous with the problem of setting the capacity of telephone switches.
Phone calls come in at random times and have random durations.
A.K. Erlang at Bell Labs began working on this problem just before 1920. From this work, a body of math called queueing theory developed. Designers use queueing theory every day in the design of their products but consistently fail to use it in the design of their development processes. Queuing Theory Queueing theory says that we can never load a stochastic process, one with any random variation in arrival rates or task durations, to 100 percent utilization. A predictable, totally deterministic process will not overload until it reaches 100 percent utilization.