Frequently, natural process times are made up of several distinct stages.
For instance, a manual task can be thought of as being comprised of individual motions (or "therbligs" as Gilbreth termed them). Suppose a manual task takes a single operator an average of one hour to perform. Alternatively, the task could be separated into 10 distinct six-minute subtasks performed by separate operators. Suppose that the subtask times are independent (i.e., uncorrelated), and assume that the coefficient of variation is 0.75 for both the single large task and the small subtasks.
Such an assumption will be valid if the relative shapes of the process time distributions for both large and small tasks are the same. (Recall that the variances of independent random variables are additive.)
(a) What is the coefficient of variation for the 10 subtasks taken together?
(b) Write an expression relating the SCV of the original tasks to the SCV of the combined task.
(c) What are the issues that must be considered before dividing a task into smaller subtasks? Why not divide it into as many as possible?
Give several pros and cons.
(d) One of the principles of JIT is to standardize production. How does this explain some of the success of JIT in terms of variability reduction?