Price, H. E.

Pulliam, R.

English

NASA Technical Reports Server

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The Allocation of Functions 
in Man·Machine Systems 
by H.E. Price 
and R. Pulliam 
BioTechnology. Inc. 
Space systems as they now exist would be impossible without automated control. We have 
become accustomed to systems which make maximum use of computer logic to control vehicles and 
ground systems. In such systems, at their best, computers are able to unburden the operator, to deal 
with complex computations, to organize information for display, andlo act with great reliability 
and speed. Many control problems can be solved in no other way. But automated control is no' 
panacea. Computers cannot set objectives, and they prove to be poor substitutes for man in 
processes such as pattern recognition and fault diagnosis. They cannot deal with the unexpected, 
nor can they construct innovative solutions to an emergency condition. Sometimes computer 
applications create, rather than solve, problems; and in operational situations we repeatedly observe 
that operators or pilots elect to defeat their automated systems, so that they themselves can assume 
manual control. 
In many of these cases the problem is an improper allocation of functions between man and 
machine. Allocation decisions were cast in hardware or software during design, and may now 
permanently limit the usability of the system. When functions are automatic the human operators 
may be unable to see what is happening or to exercise useful control. On the other hand, when 
functions a"e manual, the users may be forced to perform unnecessary chores or to do tasks for 
which hur.Jns are poorly adapted. To some extent, such design errors happen because, during 
design, there has been no deliberate consideration of which functions should be allocated to man 
and which to the machine. 
An Historical Study 
This problem is widely recognized in military and industrial settings, as well as in the aerospace 
community. Our company, BioTechnology, Inc. (BTl), recently completed a study for the Depart. 
ment of Defense in which we examined the R&D literature and the histories of recent systems 
procurements. In spite of DOD regulations which specifically require allocation of functions as a 
step in the design cycle, we could not find a single case in which the allocation of functions was 
decided, system.wide, in a systematic way. This is true, we believe, because there is no recognized 
methodology for allocating functions. Accordingly, BTl recommended the development of a 
framework and a set of methodological tools which a design team could use in allocating functions 
to man or machine. 
Developing a Method 
BTl is now developing such a framework and tools for use in nuclear power plant (NPP) design. 
In an effort supported by Oak Ridge National Laboratories and the Nuclear Regulatory Com· 
mission, BTl has developed a COh.l!eptual method for allocating functions (or assessing existing 
allocations) in NPP control rooms. The method is applicable both to earlier technology using 
electromechanical process control and to later technology exploiting the computer. 
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https://ntrs.nasa.gov/search.jsp?R=19850070377 2020-03-20T17:08:28+00:00Z
BioTechnology. Inc. 
BTl first examined the history of technology in this perspective, and then reviewed major 
models and methods which have been proposed for the allocation of functions. These begin with 
the "listing" approach. In 1951, Fitts proposed a table listing the differing capabilities of machines 
and man, to be used in support of decisions about automation. Since then, more elaborate lists have 
been suggested, for instance by Mertes and Jenny (1974), Edwards and Lees (1974), and Swain 
(1980). More elaborate simulations, procedural guides, and information support systems have 
also been developed, including HEF AM (Connelly & Willis, 1969), CAFES (Parks & Springer, 
1967), SYSSIM (Ireland, n.d.), SAINT (Workman et al., 1975), HOS (Strieb & Wherry, 1979), and 
the Hypothetical.Deductive Model of Price and Tabachnik (1968). Several of these have features 
which might be applied in determiriing functions for nuclear power plant control, but most of them 
either were never developed in an operational form, or assumed the availability of large bodies of 
reference data which do not yet exist. In spite of widespread concern, there appears to be no 
instance of a proven methodology for allocating functions to man or machine. 
Findings of this research included a recommended general, iterative procedure for allocating 
functions in the design of NPP control rooms, and some "lessons learned": 
• There has been no successful system.wide use of an allocation method. 
• Most methods for allocating functions are helpful for psychomotor tasks, but not for the 
cognitive tasks which are central to nuclear and aerospace operations. 
• Allocation of functions is like engineering design: it is an iterative process that requires 
repeated cycles of preliminary design, test, and modification. 
• Engineering design depends on an institutional memory, within the profession, of past 
successes and failures. We need such a memory for allocation (and for other human factors) 
decisions. 
• Allocation decisions drive related requirements for training, procedure writing, and per· 
sonnel selection. 
• A major need in automated systems is for man·computer communications: a means by 
which (1) the operators can remain aware of system states, even when computers exercise 
control, and (2) the computer can be informed of human interventions, including what 
those interventions are expected to accomplish. 
BTl proceeded to (1) elaborate a practical, step.by.step, reproducible method by which 
allocations can be made, and (2) identify criteria sets to be used in applying the method. The 
method will now be fully developed and applied to a selected real case in the NPP industry. 
The Recommended Procedure-Hypothesis 
A procedure was developed which differs from earlier schemes in at least one major feature: 
earlier procedures provided hypothetical solutions only. However sound they were, they provided 
only an untested hypothesis as to the correct allocation of functions. The BTl procedure added 
deductive (or empirical) tests of the hypothetical solution. Furthermore, specific tests were 
followed by closed feedback loops, so that the method can search heuristically toward an optimized 
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BioTechnology, Inc. 
man-machine interaction. The method is designed to be applied continuously, throughout the 
system design process, and to provide a series of iterative approximations approaching the goals 
expressed in a system requirements statement. 
Exhibit 1 illustrates principal steps of the proposed method_ Note the median dashed line, 
which separates an initial hypothetical analysis from the following evaluation phase. This second 
phase is called the "deductive" phase when deductive rather than empirical tests are employed, as 
must be the case during early (concept or preliminary) design phases. 
HVPOTHESIS 
PHASE 
EVALUATION 
PHASE 
Exhibit 1 
The Allocation-of-Functions Process 
DUIGN REOUIREMENTI 
• CO .. , PE .. 'O .. M ... NCl 
• LlVEL Of "'UTOM"TION 
• UTlLIT ..... IAN PE .. 'O .. MANCl 
• PSYCHOPHYSICAL CAPACITIES } 
t-.. _rO ... E~' ... IN_E_",: ~u:!: ~~':ir~ CONSTIIAINTS !-__ .;.;NO~A~~'~r~"I~I~=,L_E _'. 
\ I V 
FIAllILE 1IAMo ..... TlD FUNCTION 
"'." .... ED DIllON ... 0 
IS AlITOMATlON 
ACHIEVABLE? 
IYITIM DlIIONnAIIC ANALYSIS 
0" AND" CONNECTOR 
In the procedure, initial decisions identify these functions which must be allocated to man or 
machine for obvious reasons. Such allocations must be made to automation (Step 1) for instance, 
when regulation or policy requires it, when hostile environments preclude the presence of man, or 
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BioTechnology. Inc. 
when the required system reaction times exceed human response limitations. Allocations to human 
control (Step 2) may be mandatory, for instance, when there is a requirement to develop strategies, 
to detect patterns or trends, or when meaning or values must be assigned to events. Additional tests 
are applied for economic and technical feasibility (Step 3), and in some cases a tentative decision 
may have to be fed back for reconsideration at the system requirements level. 
Steps (1) and (2) are repeated first at the whole-system level, then for subsystems, and finally 
for portions of subsystems until those parts of the system which clearly must be controlled by man 
or computer have been partitioned off and allocated properly. Normally, this will leave substantial 
portions of the system, and of the operating procedure, which can reasonably be allocated either to 
man,"to machine, or to some combination of the two. At Step (3), these functions are classified 
according to a performance taxonomy and allocated on a best-choice basis. This process is reported 
in detail in NUREG/CR-2623 (Price, Maisano, & Van Cott, 1982). At each point in this process 
decision aids are provided, but the actual decisions remain judgmental. It is suggested that the 
procedure be applied by a team including at least one experienced human factors engineer and one 
control engineer. The method provides an orderly decision procedure and a set of decision aids 
which includes some representative quantified human performance data. Most importantly, it 
provides for documentation of the decision process. This documentation makes it possible for 
allocations decisions to be communicated widely within the systems design organization. It provides 
a basis for the evaluation steps which follow. Finally, it provides a basis for iterative improvement 
and elaboration of detail in the man-machine relationship, and interaction with engineering design 
decisions as the system design evolves. 
The Recommended Procedure-Evaluation 
At this point in each cycle of the system design, an allocation of functions to man or machine 
has been hypothesized. In a design which has reached the mockup or prototype phase, an empirical 
Itest is appropriate. But a set of deductive tests are provided as well, which can be used during 
concept formulation and other early design phases. 
First (Step 4), those functions hypothesized as "man-rated" are reviewed in detail against the 
known psychophysical capabilities of man, against system constraints, and against reliability 
requirements. If found feasible in these tests, a next step (Step 5) asks whether the human job, as 
it is emerging, is acceptable to an operator. Modifications are made at this point to ensure that 
operators will feel supported and important, that the job is coherent, and that it will fit into a 
reasonable authority and social structure. Finally, depending on outcomes of tests (Steps 4 and 5), 
elements of a preferred man-machine design are provided to systems engineering (Step 6) or are fed 
back to other steps of the design process. 
Although the work discussed in this paper is being directed at nuclear power plant operations, 
the allocation-of-functions lessons learned, method, and criteria should be applicable to many 
design issues in space systems. In both cases, the key lesson to be learned is that man and machine 
should not be considered as competitors but as complementary components for achieving system 
performance. 
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References 
Connolly, M.N., & Willis, J .E. The development of a human effectiveness function allocation 
methodology (HEFAM) (Research Memorandum SRM 70·11). San Diego, CA: Naval Personnel 
and Training Research Laboratory, October 1969. 
Edwards, E., & Lees, F .P. The influence of the process characteristics on the role of the human 
operator in process control. Applied Ergonomics, 1974,5(1),21.25. 
Pitts, P.M. (Ed.). Human engineering for an effective air·navigation and traffic·control system. 
Columbus, OH: Ohio State University Research Foundation, March 1951. 
Ireland, F.H. SYSSIM-A system simulation program for use on a time· shared computer. Moores-
town, NJ: Systems Engineering Missile and Surface Radar Division. 
Mertes, F. & Jenney, L. Automation applications in an advanced air traffic management system-
Volume HI: Methodology for man·machine task allocation (Report No. DOT·TSCOST·74-14· 
IH). McLean, VA: TRW Incorporated, August 1974. 
Parks, D.L., & Springer, W.E. Human factors engineering analytic process definition and criterion 
development for CAFES (Report No. D1S0·1S750.1). Seattle, WA: Boeing Aerospace Company, 
Research & Engineering Division, January 1976. 
Price, H.E., & Tabachnick, B.J. A descriptive model for determining optimal human performance 
in systems-Volume m: An approach for determining the optimal role of man and allocation 
of functions in an aerospace system (including technical data appendices) (NASA CR·S7S). 
Chatsworth, CA: Serendipity Associates, March 1965. -
Price, H.E., Maisano, R.E., & Van Cott, H.P. The allocation of functions in man·machine systems: 
A perspective and literature review (NRC NUREG CR.2623). Oak. Ridge, K Y : Oak. Ridge 
National Laboratories, June 19S1. 
Strieb, MJ., & Wherry, R.J., Jr. An introduction to the Human Operator Simulator (Technical 
Report 1400.02·D). Willow Grove, PA: Analytics, December 1979. 
Swain, A.D., & Guttmann, H.E. Handbook of human reliability analysis with emphasis on nuclear 
power plant applications (NRC NUREG CR·127S). Albuquerque, NM: Sandia Laboratories, 
April 1980. 
Wortman, D.B., Sigal, C.E., Pritsker, A.B., & Seifert, D.J. SAINT H documentation manual (Report 
No. AMRL.HESS·74-4). West Lafayette, IN: Pritsker & Associates, Inc., December 1975. 
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The allocation of functions in man-machine systems

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