Information Technology Reference
In-Depth Information
The Law of Requisite Variety
“states” in which a
system can exist. An electrical switch has a variety of two: on and off. A
cup of tea may have three distinct states: hot, tepid, and cold. Over time,
the cup of hot tea may have only two states — tepid and cold — and
then only one state — cold. The variety of the cup of tea has decreased
with time. Still, for some other purposes, the cup of tea could be consid-
ered to have a variety of two: strong and weak tea. As such, fundamentally,
variety is not an intrinsic property of the cup of tea. It is driven by the
ability of the receiver or regulator to distinguish between the states. Not
all possible combinations must be considered — only those that exist and
can be distinguished need to be taken into account. If a car is categorized
by a vector age, power, color, each of which can take two values old/new,
high horsepower/low horsepower, black/white, but of the eight possible
combinations, only four are actually manufactured, then the variety of the
set of cars is four, not eight.
William Ross Ashby (1903-1972) was a pioneer in the study of complex
systems. William Ross Ashby's Law of Requisite Variety states that “only
variety can destroy variety.” This law is one of the most general systemic-
cybernetic principles. It is useful in understanding any type of system.
Simply stated, it means that for any system to regulate (manage) another
system or its environment, it must have at least as much variety (com-
plexity) as the system it is trying to control. In the aforesaid car example,
once the variety is known, the manufacturer would recognize the need
for four different kinds of trucks to transport the cars, or four different
sales strategies to sell them. Similarly, if an appliance such as a laptop
computer is expected to work with both 110 Volts and 230 Volts input in
two different countries, it must have a voltage regulator that works with
110 Volts and also with 230 Volts.
Consider a project as a system that needs to be regulated, and the
project management team (architects, designers, managers) as the regula-
tors. Ashby's law implies that the regulators must have the requisite variety
to tackle its counterpart variety that exists in a project for certain vectors
such as time, budget, scope, and quality. A project manager may wish to
manage the budget of a project taking its variety as only two: over-budget
and under-budget. That is, he or she is interested in the budget being in
only two states — over or under the budget. He or she may then put in
place control plans for each of these two states (“Escalate to management
when project goes over budget”). Another project manager may, however,
decide that the budget has a variety of three: 0 to 80 percent, 80 to 100
percent, and over-budget, with a separate control plan for the 80 to 100
percent state (e.g., “Call a special review meeting when the project has
The term “variety” represents the number of
distinct
