1. ISA Transactions 39 (2000) 3±13
www.elsevier.com/locate/isatrans
Editorial viewpoint
The century's greatest contributions to control practice
R. Russell Rhinehart*
School of Chemical Engineering, Oklahoma State University, Stillwater, OK 74078-5021, USA
Flowing with the tide of re¯ection and forecast- 1. Survey participants
ing as the century changes, I took the liberty to
conduct a survey of the century's most important The participants included about 100 profes-
contributions to the practice of ``control''. I quite sionals, chosen to represent both industry and
enjoyed considering the importance of the past academe, to represent a wide range of engineering
events, and forecasting which contributions will be disciplines (Aeronautical, Chemical, Computer,
important in this new century. So did many of the Electrical, Instrument, and Mechanical), and to
survey respondents. I hope that you will enjoy this represent a wide range of perspectives (vendors,
re¯ection, and also ®nd it useful. users, researchers, developers, and managers).
This was a non-scienti®c survey, with a limited This balance of people mainly came from those
number of participants. However, the participants who are active in ISA Transactions and the
are all established experts, who represent a wide American Automatic Control Council. From the
variety of disciplines and professions, and who academics, I selected those with a strong applica-
appear to have submitted well-considered opi- tions orientation. From industry folks, I selected
nions. Accordingly, I believe the results are fairly those working at the leading edge. I classify the
legitimate, and the diversity and number of parti- participants as either practical academics or
cipants makes the results free of marketing bias. advanced practitioners. For the most part, the
For those who are directing the development of academics and practitioners had similar views of
textbooks, handbooks, degree programs, continu- the relative importance of the contributions.
ing education programs, top elements in the sur- I also selected senior colleagues, so that their
vey will be useful for specifying content. Those retrospective on the century will be more com-
same elements will be a useful guide for those plete. However, even the most senior gurus of
directing their own professional development. For today only experienced their profession in the last
those who are planning and managing the techni- third of the century. This creates a perspective
cal future of an operating company, a vendor, or a bias. For example, when using symbolic pro-
service provider, this survey contains a useful cessors and spreadsheet macros, it is hard to
vision of the most promising and practicable new appreciate the signi®cance of the slide-rule. It dis-
technology. appeared about 25 years ago; but until then, was
an indispensable tool. Many of us never experi-
enced its importance; and even for those of us who
* Tel.: +1-405-744-5280; fax: +1-405-744-6338. did, the computer has trivialized our remembrance
E-mail address: rrr@gibbs.cheng.oksate.edu of the slide-rule's importance. Respondents were
0019-0578/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0019-0578(00)00010-0

2. 4 R.R. Rhinehart / ISA Transactions 39 (2000) 3±13
instructed to be aware of this; but even so, this process, or they manipulate the process.
survey must have a bias for the last quarter of the They include hardware as well as software.
century. 4. Control, Decision, and Communication
Instrumentation Ð These contributions, either
hardware or software, are the ones that
2. Survey categories implement the control strategy and automate
the decision.
The survey started with a small, balanced group 5. Organization Ð These items included orga-
of people to help create both the scope and the nizations of people (such as ISA and NASA)
de®nition of ``the practice of control''. This would that contributed to control practice by stan-
guide our selection of the most signi®cant con- dardizing, demonstrating, and disseminating
tributions, which come from a variety of distinctly techniques. The list also includes books, an
di€erent directions. Theory contributes to practice organization of technology that was sig-
when it directly leads to a tool that comes into ni®cant to acceptance and use.
widespread use. Top examples included frequency
analysis that led to rules for controller designing Considering this scope of contributions, these
and tuning, and state space analysis that led to statements guided selection for the ``most sig-
modern control techniques and the Kalman ®lter. ni®cant contributions to the practice of control'':
Instrument devices were included, with some being Include all aspects of ``control'' Ð from sensing
the pneumatic-mechanical PID wonder, the gas to system optimization, from technology to
chromatograph, the Norden bombsight, the people. To focus on the practice, consider where
pneumatic valve actuator, and the Coriolis ¯ow- non-control specialists use ``control'' Ð where it
meter. Of course, the computer and communica- is a useful tool to make something work. To
tion systems were included as tools that ®nally focus on the most important contributions, con-
allowed data access, the automation of process sider both the tangible bene®t to human welfare
analysis and higher level logic, nonlinear control, that was achieved, the degree of acceptance by
and optimization. Control strategies such as mul- those who use control as a tool, the paradigm
tivariable model predictive control and fuzzy logic shift that resulted from the contribution, or the
control were included. Finally, organizational magnitude of the implementation need that was
events such as textbooks and professional societies satis®ed.
that revealed and standardized the practice of The ``demonstrated utility'' aspect of the criteria
control were added. Contributions came from a creates a bias against contributions of the past 5 to
wide range of in¯uences. We developed a list of 99 15 years that are just now developing into sig-
items. It was a great century! ni®cance. So, we created a sixth category:
Considerations of the diversity of items on the
list led to grouping of the contributions into 5 6. Most Promising Newcomers Ð These items
categories: included ones that were introduced later in
the twentieth Century, which have consider-
1. Concepts and Theory Ð These contributions able promise and signi®cant isolated demon-
were signi®cant because they formed the strations, but which have not had the time to
basis for techniques and products that made come to full fruition within the practice.
signi®cant impact on the practice of control.
2. Techniques Ð These items are the procedures
and algorithms for applying, designing, or 3. Survey stages
analyzing elements of the control system.
3. Process Instrumentation Ð These items are The survey had three phases. First, about 10 of
the ones that ``touch''' the process. They us de®ned the practice of control. Then I sent the
provide or convey information about the de®nition and some starter examples to about 100

3. R.R. Rhinehart / ISA Transactions 39 (2000) 3±13 5
colleagues and asked them to suggest more items. perhaps should have been placed within the New-
About 35 people responded. I used the initial comers category. Interestingly, it was one of a few
small group to help re®ne and organize the items. items where there was a moderate di€erence in the
Table 1 shows the entire list. Finally, I sent that importance attributed by academics and practi-
list to each of the 100 and asked them to choose tioners. I have spent half of my career in industry
their top 3±5 in each category. There were about and half in academe, and am interested in view
16 items in each category, some of which might point di€erences of the two careers. Academics
dominate the others; so, allowing 3±5 choices in rated Internal Model Principle high. I will let you
any category would permit strong acknowl- decide whether this is a useful technology that has
edgment of the second-place items. Forty-two not been fully discovered by industry or an analy-
folks returned feedback in time for this editorial to tically rich concept with little utility. I believe the
go to press. Table 1 lists the raw data. Fig. 1 shows issue is worth investigating.
that the mix was 44% academe and 56% industry. Fig. 3 displays the top items in the Techniques
I appreciate all of those who participated. category. In my view each of these items de®nes
skills that a control engineer should have, and
items on this list will continue to be important in
4. Survey results the twenty-®rst century. This list should guide
training and educational programs. If votes/per-
Figs. 2±7 display items in each category that son is an indication, this was the most important
collectively received about 75% of the votes. Items category to the practice of control. Instructions
that received only a few votes were deemed to be were to ``choose 3±5 items from each list''; how-
signi®cant by a few people. They may even form ever, individuals averaged choosing 5.4 items per
the basis for a strong segment of the industry, but person in this category! That's right, many people
they did not share the collective, widespread ar- chose 6 items. Some, 7 items. Only a few sub-
mation. Shown in Fig. 2, the top items in Category mitted just three.
1, Concepts and Theory were the classic feedback The item on Fig. 3 called ``Classical Advanced
principle (bias some aspect of the control action Control'' includes the 30's and 40's SISO techni-
with the error and a cumulative sum) and fre- ques of ratio, cascade, feedforward, override,
quency analysis. These originally supported the decouplers, and reset feedback. Industry folks
SISO PI control of the 20H s that continues as the rated them very important (second most, from the
backbone of control today. The importance of the list of 20 items); academics did not (12th on their
feedback principle is evident in its number of list). For most of this study the industry and aca-
votes, 40, re¯ecting that 95% of the respondents demic views were consistent. However, this was
chose it as one of their top 3±5 from the list of 11 one of the two cases of statistically signi®cant
items. The other top items in the Concepts and (a=0.05) di€erences between academic and
Theory category are those that structure MIMO industry opinions on the 99 items in this survey. It
control of the computer age: State-Space Analysis, appears that these SISO techniques will continue
Digital Control Theory, and Optimal Control to be fundamental to process automation. They
Theory. If votes per person per item is an indica- comprise the majority of ``advanced'' control
tion of importance, this was the most important loops today. Industry is ``slow'' to upgrade when
category at 0.39 votes/person/item. technology at hand in older equipment is fully
Just short of making the top of the list was sucient, cost e€ective, and understood by
``Internal Model Principle''. In my view, Internal operators. Since these SISO techniques are stan-
Model Control is a useful SISO tool. From its dard o€erings on contemporary DCS and PLC
uni®ed view, the concept leads to PID, Smith pre- equipment, they are guaranteed to be a workhorse
dictor, and other control strategies. It has also of process control well into this century. Perhaps
lead to the ``lambda tuning'' methods. It is an 80H s the vote discrepancy is simply a re¯ection of respon-
development within the academic community, and dent demographics. The industry respondents were

4. 6 R.R. Rhinehart / ISA Transactions 39 (2000) 3±13
Table 1
Survey results on items suggested as having the greatest contributions to the practice of control in the twentieth century
Number of Number of Total
academe votes industry votes votes
Category 1 items
Variational calculus 1 1 2
International practical temperature scale 0 3 3
Reliability methods for safety 0 4 4
Strategy design by ®rst-principles models 2 8 10
Internal model principle 7 4 11
Least squares system identi®cation 4 7 11
Optimal control 8 9 17
Digital control theory 8 11 19
State space and modern control theory 15 15 30
Frequency response analysis 15 19 34
Feedback principle 17 23 40
Category 2 items
Self-tuning regulators 0 1 1
Discrete event approaches 0 1 1
Envelope control 0 2 2
Cohen & Coon tuning 0 3 3
Predictive maintenance/reliability-centered maintenance 0 3 3
Heuristic tuning rules 1 5 6
Bristol's relative gain array 3 3 6
Deadtime compensators 4 3 7
Valve sizing 0 8 8
Dynamic programming 3 5 8
Statistical process control 2 8 10
Linear-quadratic optimal control 6 6 12
Classical advanced control 2 14 16
Nyquist stability criterion 10 7 17
Root locus design 9 9 18
Simulation, CAD 6 13 19
Multivariable MPC 7 13 20
Bode design 11 9 20
Kalman/kalman-Bucy ®lter 11 11 22
Zeigler-nichols tuning 9 17 26
Category 3 items
Bailey boiler meter 0 0 0
Ultrasonic level sensors 0 0 0
pH sensor/transmitter 0 1 1
Nuclear level and density sensing 0 1 1
Miniaturization of pneumatic instruments 2 0 2
Weigh cells 0 2 2
Miniature electronic sensing instruments 1 1 2
Toxic and hazardous gas detectors 0 2 2
Flame scanners 0 3 3
Bimetallic thermostat 1 2 3
Bourdon tube 3 1 4
Float control 4 3 7
Equal % valves 3 4 7
Coriolis ¯owmeter 2 6 8
2-Wire control loop 3 6 9
I/P transducer 2 7 9
Composition analysis 1 9 10
Wireless technology 4 6 10
(Table continued on next page)

5. R.R. Rhinehart / ISA Transactions 39 (2000) 3±13 7
Table 1 (continued)
Number of Number of Total
academe votes industry votes votes
Robots 6 4 10
Thermostat 7 3 10
Gyroscope 7 4 11
Fiber optics 6 7 13
Valve positioners 4 10 14
dp cell 2 13 15
Pneumatic valve 4 17 21
Category 4 items
Miniature electronic controllers 0 1 1
SCADA 0 3 3
Norden bombsight 1 2 3
Centralized control 2 1 3
Logic solvers 1 2 3
Passive zenier barrier devices for intrinsic safety 0 3 3
Analogue computers 2 2 4
Sequential automation 0 4 4
Watt regulator 2 2 4
Voting architecture 0 4 4
DDC 4 3 7
CRT displays for HMI 3 7 10
Distributed control 3 9 12
Op-Amp (operational ampli®er) 9 11 20
Programming languages 5 16 21
Transistor 6 19 25
PID controller 15 20 35
Microprocessor 17 24 41
Category 5 items
Wilkinson's book 1 3 4
Danzig's book 2 5 7
Eckman's book 5 5 10
Black's book 3 10 13
Weiner's book 6 8 14
Shinsky's book 5 11 16
P& id Symbol standards (ISA and SAMA) 5 12 17
IFAC 12 5 17
IEEE 10 10 20
ISA 9 13 22
NASA 8 14 22
Category 6 items
Auto tuning by relay feedback 2 2 4
Sliding mode (variable structure control) 3 4 7
Enterprise-level management systems 2 6 8
Optical computing 3 5 8
Connectivity tools, networks 1 8 9
Nonlinear model predictive control 6 7 13
Robust inferentials 5 9 14
Soft sensors 3 12 15
Object oriented programming 5 10 15
Open systems 6 10 16
Fuzzy logic 11 7 18
Adaptive, self-tuning controllers 8 11 19
Intelligent systems 10 10 20
Neural networks 10 11 21

6. 8 R.R. Rhinehart / ISA Transactions 39 (2000) 3±13
Traditional control theory places the dynamics
and faults of sensors and ®nal control elements
within the mysterious ``plant''. But, Process
Instruments containing the most number of items
in this survey suggests that they are very impor-
tant. Perhaps we should explicitly account for the
in¯uence of these items in our analysis of control
systems.
Fig. 1. Distribution of respondents.
Fig. 5 presents the top items in the instruments
used for control, decision, and communication.
The ``microprocessor/computer'' was voted as the
predominantly from the process industry where most important contribution to control from this
these techniques are particularly important, while category, and considering that it received 41 votes
academic respondents represented a wider variety out of 42 voters, it is the most highly rated item
of disciplines. These data cannot be de®nitive, but from this entire study. It is the tool that allows all
they suggest that the academic community should of the advanced practices in control, communica-
consider the value of ``Classical Advanced Con- tions, optimization, smart instruments, etc. At
trol'' within the engineering curriculum. least a few of the respondents included with it the
Fig. 4 shows the top items in the Process mass storage devices that are integral to the com-
Instrumentation category. I believe that the vari- puter. This is one of the few items in this study
ety of items and the mixture of applications that was not originally created for control. Oddly,
represented on Fig. 4 arm the breadth of dis- it has become the most important. Considering the
ciplines participating in this survey. This category computer applications throughout the other seg-
had the most number of items, 25, perhaps indi- ments of our life, the ``microprocessor/computer''
cating the importance of the contribution of sen- could become known as the greatest contribution
sors and ®nal control elements to control. to all human endeavors of the century.
Fig. 2. Top concepts and theory (lower bar academe, upper bar industry).

7. R.R. Rhinehart / ISA Transactions 39 (2000) 3±13 9
While ``Programming Languages'' and the was ``Programming Languages Ð high level over
``Transistor'' were not as unanimous a choice as machine instruction''. I had not considered pro-
the microprocessor, they are of signi®cant impor- gramming languages as a major contribution to
tance. And, like the computer, neither was created the practice of control in my voting, and chose
speci®cally for control. The full name for voting other items. But, with the collective opinion of so
Fig. 3. Top techniques (lower bar academe, upper bar industry).
Fig. 4. Top process instruments (lower bar academe, upper bar industry).

8. 10 R.R. Rhinehart / ISA Transactions 39 (2000) 3±13
many gurus, in retrospect I have to agree that high challenge our views against the collective opinions
level language has permitted the use of simulators, presented by this body of experts.
on-line analysis, custom displays, fuzzy logic, and Note that the PID controller was a strong sec-
much more. Certainly, it should be rated as a sig- ond on Fig. 5. When I ®rst saw one, the mystifying
ni®cant contribution. Perhaps all of us could pneumatic-mechanical workings, the elegant brass
Fig. 5. Top control decision and communication instruments (lower bar academe, upper bar industry).
Fig. 6. Top organizations Ð of people and information (lower bar academe, upper bar industry).

9. R.R. Rhinehart / ISA Transactions 39 (2000) 3±13 11
bellows and levers, the clock-works delicacy of its physical measurement from the operator interface
internals, simplicity of drivers, and clever device. Since the operator interface no longer had
mechanisms for display and tuning immediately to support the rugged measuring device (bourdon
enchanted me. Regardless of the romance of such tube, thermal system spiral, etc.), the operator
devices, and regardless of their (almost) replace- interface could be reduced in size to typically
ment by analogue or electronic equivalents, the 6HH HÂ3HH W. This permitted a much greater density
pneumatic PID instrument moved control from of information display, up to 48 variables per foot
governors (with manual bias adjustment to of panelboard. This greatly increased the number
remove o€set) to truly automatic. Their contribu- of variables, therefore, the scope of the process,
tion to the practice was momentous. that one operator could monitor. Some installa-
Harold Wade, of Wade Associates, adds that tions incorporated a graphic diagram of the pro-
the miniaturization and standardization of the cess into the control panel, with miniature
pneumatic devices was also a signi®cant event: indicators/recorders/controllers placed symboli-
``Miniaturization freed the industry from reliance cally in the proper relation to the point of mea-
on directly connected devices for measurement, surement of the process variable. Control rooms
indication, recording, and control. Prior to this became more centralized, resulting in better coor-
most control rooms consisted of a control panel with dination of information between process units.''
one or two rows of large case recorder-controllers, Direct Digital Control (DDC) fell just short of
near to and directly connected to the process. A making an appearance in Fig. 5. It was in the top
typical instrument size was 16HH HÂ12HH W; each cumulative 80%, but I only showed the top 75%
instrument provided one to three measurement in the ®gures. As I recall, DDC was the ®rst com-
pens. This allowed an operator to monitor 4±6 mercial entry of the computer into process con-
variables per linear foot of panelboard length. trol. It was short lived, as the evolved PLC and
This greatly restricted the scope of the process for DCS products overcame reliability and concept
which one operator could maintain surveillance. problems. It was an experiment that did not quite
Pneumatic signal transmission (®nally standar- work, and I suspect that the short life and rela-
dized at 3±15 psig) permitted separation of the tively primitive technology of DDC resulted in its
Fig. 7. Most promising newcomers (lower bar academe, upper bar industry).

10. 12 R.R. Rhinehart / ISA Transactions 39 (2000) 3±13
relatively low ranking on this list. Here, ``demon- Standards'', which uni®ed communication among
strated utility'' was a measure of rank, and it practitioners, was highly rated as a signi®cant con-
seemed to overshadow ``paradigm shift''. Cer- tribution. Statistically, the most signi®cant di€er-
tainly, however, the vision and bold moves of ence of opinion between academic and industry
those who accepted the DDC deserve respect. folks in this survey was on the importance of the
Along the lines of that analysis, all of the items International Federation of Automatic Control.
listed in Table 1, not just those making the ®gures, Again demographics could have been the reason.
have legitimate claim to being substantial con- Academics were distributed around the world.
tributions to control in the twenthith Century. Industry folks were almost exclusively from North
Fig. 6 presents the top items on the organization Ð America. However, this may signal a message,
of people and information list. The US National ``Keep an eye on IFAC,'' to industry.
Aeronautics and Space Administration (NASA) ``Control'' has its landmark books; those valued
was the top vote getter. This also surprised me. In as such a useful guide to technology that they ele-
my process engineering view, I do not see con- vated the practice. The respondents placed F. G.
tributions from the space programs. However, as Shinskey's book ``Process Control Systems'', and
Al Comello commented on his votes, NASA ``F F F Nobert Wiener's book, ``Cybernetics, or Control
gave a stage for applying control concepts and a and Communication in Animal and Machines'' on
platform for the birth of sensors/devices that did the top of the list. If you have a copy of one of
not exist. Industry and academia could not have these, or any of the books listed in Table 1, better
hoped to deliver this under an economic model. keep it. Museums may want them!
NASA provided a marketplace and a relatively Results from the ®nal category, ``Most Promis-
universal access to the technology.'' Al is a mem- ing Newcomers'', are presented in Fig. 7. Each of
ber of the Editorial Advisory board for ISA these items has a record of credible practice appli-
Transactions. cations. For each there are several companies
In retrospect it is odd that business organiza- selling products. These are not ``futuristic'' imagi-
tions were not included on the list. Private enter- nations, but recent technology, fully commercial
prise invested substantial development e€ort to today, deemed to have great potential, but not
create products that implemented the techniques. enough time to establish itself as a ``greatest con-
They brought to us the instruments and the com- tribution''. These items may become known as the
mercial software products for data processing, twentieth Century's great legacy for the twenty-
control, and optimization. I suspect that none of ®rst. I believe that we should look at these items as
us who created the survey were thinking along the we plan for the future.
lines that allowed us to ``see'' the contributions of ``Neural Networks'' (NN), voted top, is a ¯ex-
the commercial organizations during the brief ible method for empirical modeling. In the 80H s,
period of creating the list. As I consider the mag- hype-driven, often inappropriate applications of
nitude and diversity of products, I have to apol- primitive NNs gave NNs a bad initial reputation.
ogize for not including business names on the However, note that both the academic and indus-
survey. However, I am somewhat relieved at the trial participants of this survey placed NNs as a
oversight, for there are too many to mention most promising newcomer to the practice of con-
within the page limit of this article. trol. Since NNs are often the tool that is the basis
Note that the bar heights in Fig. 6 are fairly of other top items in this category (Nonlinear
uniform. It seems that many organizations are Model Predictive Control, Soft Sensors or Virtual
perceived as having made fairly equivalent con- On-Line Analyzers, Robust Inferentials, and
tributions. The professional societies of ISA, Intelligent Systems), perhaps the importance of
IEEE, and IFAC certainly have contributed by NNs should be elevated.
dissemination of useful information through con- ``Intelligent Systems'' are software programs that
ferences, exhibitions, continuing education cour- ``learn'' and ``modify'' their action. So do ``Adap-
ses, and standards. Notable, the item ``Symbol tive and Self-Tuning Controllers''. In a too-simple,

11. R.R. Rhinehart / ISA Transactions 39 (2000) 3±13 13
they-will-jump-on-me-description, I'll o€er the that it o€ers insight for further contemplation. I
following description of the di€erence. Intelligent hope that it has had the same impact for you.
Systems use ``unstructured'' non-equation-based The list in Table 1 is not complete. For instance
techniques such as NNs and genetic algorithms, the recent and important advances in batch man-
and are often called agents. Presently, their main agement automation, continuous improvement,
demonstrations are in games and computer- system reliability, the Internet, and data management
against-computer contests, but success is evident. were not included. Survey respondents pointed
The extension to process management is obvious. this out to me in their votes. I am looking forward to
Adaptive and Self-Tuning Controllers are equa- see how it all sorts out in the twenty-®rst century.
tion-based. The adaptation is very structured.
They, too, have substantial proven applications.
``Fuzzy Logic'' was high on the list in spite of a
relatively low number of industrial votes. In my
view Fuzzy Logic Control (FLC) o€ers a relatively
easy way to get a computer to take the same
heuristic action that an experienced engineer or
operator would take. Often the human intuitive
intervention (®ne-tuning, control, balancing,
model adjustment, or optimization) of a system is
better (cheaper, more robust, e€ective) than any
attempt by a model-based system. FLC has a
multitude of successes within both the process
industry and in the OEM. Why then is there a
discrepancy in the academic and industrial per-
ception of its importance? Perhaps a reason is jar- Dr. R. Russell Rhinehart is the Head of the
gon. Computer and human cognitive scientists School of Chemical Engineering at Oklahoma
developed fuzzy logic, and they must have enjoyed State University, and has considerable experience
coming up with names like ``universe of discourse'' in both industry (13 years) and academe (14
to describe the simple concept of ``measurement years). Russ' primary research interests are in the
range''. I believe that the language of FLC is a practical application of advanced technology for
barrier to its application. Further, since FLC pro- automated process management (control, optimi-
grams are relatively simple to write from scratch, zation, monitoring). Necessarily, his program has
FLC may not have the promotional push by ven- a strong experimental component.
dors to generate industrial acceptance. Alter- Russ is an active member of AIChE and ISA
nately, industry may be aware of practicable and holds several professional society positions
problems with FLC that academe is not. We including Editor-in-Chief of ISA Transactions,
should work to learn of each other's view and and General Chair for the 2002 American Control
either use it, or place it lower on the list. Conference. His 1968 B.S. in Chemical Engineer-
ing and M.S. in Nuclear Engineering are both
from the University of Maryland. His 1985 Ph.D.
5. Closing in Chemical Engineering is from North Carolina
State University.
Here was an unscienti®c study from an uncon- Dr. Rhinehart's locators are: oce mail 423
trolled demographic mix presented in an editorial, Engineering North, Oklahoma State University,
not a refereed paper. It is simply one opinion of Stillwater, OK 74078-5021; Tel.: +1-405-744-5280;
the most important contributions to control of the fax +1-405-744-6338;
twentieth century. However, I enjoyed the exercise E-mail address: rrr@gibbs.cheng.okstate.edu;
as a review of where we have been, and suspect web http://www.cheng.okstate.edu.