What is a constraint?
In his book, "The Goal", Goldratt gives the example of the scout troop on
a hike. The goal of Alex Rogo as I understand it was to get the troop to
the campground in some reasonable amount of time while keeping the group
close enough together to maintain adult supervision.
The constraint was identified as Herbie, the slowest moving boy in the
troop. He was easily identified by the queue of boys behind him and
the growing space (starved queue) between Herbie and the boys in front
The term "constraint" as used in the story above is the resource within a
system which prevents system output from reaching a goal. The constraint
is easily identifiable by the queue which forms before it and the starved
resources which follow it.
The term "constraint" is used in the TOC literature in several other ways:
constraint = anything that limits the output of a system
constraint = any boundary condition.
constraint = any cause of a limiting resource
To continue with the hiking example:
Alex Rogo tried to make the best of Herbie's capability by encouraging
Herbie and by keeping breaks short. This "exploit" was not enough to
meet the goal.
The troop was subordinated to Herbie's walking speed. This kept the group
together, but it wasn't enough to meet the goal. Alex then did a cause and effect analysis to elevate the constraint and chose the root cause "heavy backpack". He then solved the problem by
distributing Herbie's supplies among the faster hikers. Alex was vigilant to again evaluate the hike for a new constraint. Luckily Herbie remained the constraint but his increased capacity was sufficient to meet the goal.
The above story uses a "capacity limiting resource" definition of the
constraint. The "capacity limiting resource" definition produced an
observable testable constraint that is not dependent upon the
thinking process of the observer. In the example above Herbie is clearly
Now here is where is where things begin to get a little fuzzy in TOC.
In TOC parlance, the cause of a resource constraint can be a "policy
constraint". Using this logic, it could be said that the true constraint
was Herbie's policy of placing large amounts of food and supplies in
his backpack. Herbie was doing a local optimization for his personal
food supply and comfort at camp. In the process he sub optimized the
We could arbitrarily choose other "policy constraints". For example,
Herbie's pack was too heavy. Suppose we decide the solution to the
problem is that packs should be less than a certain weight before
hikers are permitted to go on hikes. In this case a "policy constraint"
might have been - the Scout troop does not check pack weight before
The "policy constraints" mentioned above are causes selected at
convenient locations in the cause effect continuum. As such they
are not uniquely determined but are instead determined by the preferred
When we allow the definition of constraint to include the solution
space of the cause and effect network which is related to elevating the
capacity limiting resource, then there can be no singular constraint. It
is this type of "constraint" which is the source of much confusion.
Policy constraints are not measurable system attributes. It is not
possible to measure a policy, it is only possible to measure the
effect a policy has on a system, subsystem, or individual resource capacity.
"The constraint", when it is a "capacity limiting resource constraint" is
a measurable attribute of a system. Measurement units would typically be a
production rate. It is very much different from a "policy constraint".
In my opinion, TOC would gain considerable clarity if there were a
special term that applied only to system capacity limiting resources. In
honor of Goldratt's story about the scout troop, I propose we call
the resource that limits system output the herbie.
The herbie is easily testable and falsifiable. If a resource is a
herbie, and if we elevate the capacity of that resource, then
system capacity must increase. If we lower the capacity of the herbie,
system capacity must decrease. If we vary the capacity of any other
resource, we should expect no change in system capacity. Test construction should consider the possibility of balanced resources which would allow a reduced system capacity when the initial herbie
capacity is reduced but would result in a new herbie when initial herbie
capacity is increased.