to the Scientific Method
Wolfs, University of Rochester
I. The scientific method has four steps
II. Testing hypotheses
III. Common Mistakes in Applying the Scientific Method
IV. Hypotheses, Models, Theories and Laws
V. Are there circumstances in which the Scientific Method is not
to the Scientific Method
The scientific method is the process by which scientists, collectively
and over time, endeavor to construct an accurate (that is, reliable,
consistent and non-arbitrary) representation of the world.
Recognizing that personal and cultural beliefs influence both
our perceptions and our interpretations of natural phenomena,
we aim through the use of standard procedures and criteria to
minimize those influences when developing a theory. As a famous
scientist once said, "Smart people (like smart lawyers) can
come up with very good explanations for mistaken points of view."
In summary, the scientific method attempts to minimize the influence
of bias or prejudice in the experimenter when testing an hypothesis
or a theory.
The scientific method has four steps
1. Observation and description of a phenomenon or group of phenomena.
Formulation of an hypothesis to explain the phenomena. In physics,
the hypothesis often takes the form of a causal mechanism or a
Use of the hypothesis to predict the existence of other phenomena,
or to predict quantitatively the results of new observations.
Performance of experimental tests of the predictions by several
independent experimenters and properly performed experiments.
the experiments bear out the hypothesis it may come to be regarded
as a theory or law of nature (more on the concepts of hypothesis,
model, theory and law below). If the experiments do not bear out
the hypothesis, it must be rejected or modified. What is key in
the description of the scientific method just given is the predictive
power (the ability to get more out of the theory than you put
in; see Barrow, 1991) of the hypothesis or theory, as tested by
experiment. It is often said in science that theories can never
be proved, only disproved. There is always the possibility that
a new observation or a new experiment will conflict with a long-standing
As just stated, experimental tests may lead either to the confirmation
of the hypothesis, or to the ruling out of the hypothesis. The
scientific method requires that an hypothesis be ruled out or
modified if its predictions are clearly and repeatedly incompatible
with experimental tests. Further, no matter how elegant a theory
is, its predictions must agree with experimental results if we
are to believe that it is a valid description of nature. In physics,
as in every experimental science, "experiment is supreme"
and experimental verification of hypothetical predictions is absolutely
necessary. Experiments may test the theory directly (for example,
the observation of a new particle) or may test for consequences
derived from the theory using mathematics and logic (the rate
of a radioactive decay process requiring the existence of the
new particle). Note that the necessity of experiment also implies
that a theory must be testable. Theories which cannot be tested,
because, for instance, they have no observable ramifications (such
as, a particle whose characteristics make it unobservable), do
not qualify as scientific theories.
the predictions of a long-standing theory are found to be in disagreement
with new experimental results, the theory may be discarded as
a description of reality, but it may continue to be applicable
within a limited range of measurable parameters. For example,
the laws of classical mechanics (Newton's Laws) are valid only
when the velocities of interest are much smaller than the speed
of light (that is, in algebraic form, when v/c << 1). Since
this is the domain of a large portion of human experience, the
laws of classical mechanics are widely, usefully and correctly
applied in a large range of technological and scientific problems.
Yet in nature we observe a domain in which v/c is not small. The
motions of objects in this domain, as well as motion in the "classical"
domain, are accurately described through the equations of Einstein's
theory of relativity. We believe, due to experimental tests, that
relativistic theory provides a more general, and therefore more
accurate, description of the principles governing our universe,
than the earlier "classical" theory. Further, we find
that the relativistic equations reduce to the classical equations
in the limit v/c << 1. Similarly, classical physics is valid
only at distances much larger than atomic scales (x >> 10-8
m). A description which is valid at all length scales is given
by the equations of quantum mechanics.
are all familiar with theories which had to be discarded in the
face of experimental evidence. In the field of astronomy, the
earth-centered description of the planetary orbits was overthrown
by the Copernican system, in which the sun was placed at the center
of a series of concentric, circular planetary orbits. Later, this
theory was modified, as measurements of the planets motions were
found to be compatible with elliptical, not circular, orbits,
and still later planetary motion was found to be derivable from
in experiments have several sources. First, there is error intrinsic
to instruments of measurement. Because this type of error has
equal probability of producing a measurement higher or lower numerically
than the "true" value, it is called random error. Second,
there is non-random or systematic error, due to factors which
bias the result in one direction. No measurement, and therefore
no experiment, can be perfectly precise. At the same time, in
science we have standard ways of estimating and in some cases
reducing errors. Thus it is important to determine the accuracy
of a particular measurement and, when stating quantitative results,
to quote the measurement error. A measurement without a quoted
error is meaningless. The comparison between experiment and theory
is made within the context of experimental errors. Scientists
ask, how many standard deviations are the results from the theoretical
prediction? Have all sources of systematic and random errors been
properly estimated? This is discussed in more detail in the appendix
on Error Analysis and in Statistics Lab 1.
Common Mistakes in Applying the Scientific Method
As stated earlier, the scientific method attempts to minimize
the influence of the scientist's bias on the outcome of an experiment.
That is, when testing an hypothesis or a theory, the scientist
may have a preference for one outcome or another, and it is important
that this preference not bias the results or their interpretation.
The most fundamental error is to mistake the hypothesis for an
explanation of a phenomenon, without performing experimental tests.
Sometimes "common sense" and "logic" tempt
us into believing that no test is needed. There are numerous examples
of this, dating from the Greek philosophers to the present day.
common mistake is to ignore or rule out data which do not support
the hypothesis. Ideally, the experimenter is open to the possibility
that the hypothesis is correct or incorrect. Sometimes, however,
a scientist may have a strong belief that the hypothesis is true
(or false), or feels internal or external pressure to get a specific
result. In that case, there may be a psychological tendency to
find "something wrong", such as systematic effects,
with data which do not support the scientist's expectations, while
data which do agree with those expectations may not be checked
as carefully. The lesson is that all data must be handled in the
common mistake arises from the failure to estimate quantitatively
systematic errors (and all errors). There are many examples of
discoveries which were missed by experimenters whose data contained
a new phenomenon, but who explained it away as a systematic background.
Conversely, there are many examples of alleged "new discoveries"
which later proved to be due to systematic errors not accounted
for by the "discoverers."
In a field where there is active experimentation and open communication
among members of the scientific community, the biases of individuals
or groups may cancel out, because experimental tests are repeated
by different scientists who may have different biases. In addition,
different types of experimental setups have different sources
of systematic errors. Over a period spanning a variety of experimental
tests (usually at least several years), a consensus develops in
the community as to which experimental results have stood the
test of time.
Hypotheses, Models, Theories and Laws
In physics and other science disciplines, the words "hypothesis,"
"model," "theory" and "law" have
different connotations in relation to the stage of acceptance
or knowledge about a group of phenomena.
hypothesis is a limited statement regarding cause and effect in
specific situations; it also refers to our state of knowledge
before experimental work has been performed and perhaps even before
new phenomena have been predicted. To take an example from daily
life, suppose you discover that your car will not start. You may
say, "My car does not start because the battery is low."
This is your first hypothesis. You may then check whether the
lights were left on, or if the engine makes a particular sound
when you turn the ignition key. You might actually check the voltage
across the terminals of the battery. If you discover that the
battery is not low, you might attempt another hypothesis ("The
starter is broken"; "This is really not my car.")
word model is reserved for situations when it is known that the
hypothesis has at least limited validity. A often-cited example
of this is the Bohr model of the atom, in which, in an analogy
to the solar system, the electrons are described has moving in
circular orbits around the nucleus. This is not an accurate depiction
of what an atom "looks like," but the model succeeds
in mathematically representing the energies (but not the correct
angular momenta) of the quantum states of the electron in the
simplest case, the hydrogen atom. Another example is Hook's Law
(which should be called Hook's principle, or Hook's model), which
states that the force exerted by a mass attached to a spring is
proportional to the amount the spring is stretched. We know that
this principle is only valid for small amounts of stretching.
The "law" fails when the spring is stretched beyond
its elastic limit (it can break). This principle, however, leads
to the prediction of simple harmonic motion, and, as a model of
the behavior of a spring, has been versatile in an extremely broad
range of applications.
scientific theory or law represents an hypothesis, or a group
of related hypotheses, which has been confirmed through repeated
experimental tests. Theories in physics are often formulated in
terms of a few concepts and equations, which are identified with
"laws of nature," suggesting their universal applicability.
Accepted scientific theories and laws become part of our understanding
of the universe and the basis for exploring less well-understood
areas of knowledge. Theories are not easily discarded; new discoveries
are first assumed to fit into the existing theoretical framework.
It is only when, after repeated experimental tests, the new phenomenon
cannot be accommodated that scientists seriously question the
theory and attempt to modify it. The validity that we attach to
scientific theories as representing realities of the physical
world is to be contrasted with the facile invalidation implied
by the expression, "It's only a theory." For example,
it is unlikely that a person will step off a tall building on
the assumption that they will not fall, because "Gravity
is only a theory."
in scientific thought and theories occur, of course, sometimes
revolutionizing our view of the world (Kuhn, 1962). Again, the
key force for change is the scientific method, and its emphasis
Are there circumstances in which the Scientific Method is not
While the scientific method is necessary in developing scientific
knowledge, it is also useful in everyday problem-solving. What
do you do when your telephone doesn't work? Is the problem in
the hand set, the cabling inside your house, the hookup outside,
or in the workings of the phone company? The process you might
go through to solve this problem could involve scientific thinking,
and the results might contradict your initial expectations.
any good scientist, you may question the range of situations (outside
of science) in which the scientific method may be applied. From
what has been stated above, we determine that the scientific method
works best in situations where one can isolate the phenomenon
of interest, by eliminating or accounting for extraneous factors,
and where one can repeatedly test the system under study after
making limited, controlled changes in it.
are, of course, circumstances when one cannot isolate the phenomena
or when one cannot repeat the measurement over and over again.
In such cases the results may depend in part on the history of
a situation. This often occurs in social interactions between
people. For example, when a lawyer makes arguments in front of
a jury in court, she or he cannot try other approaches by repeating
the trial over and over again in front of the same jury. In a
new trial, the jury composition will be different. Even the same
jury hearing a new set of arguments cannot be expected to forget
what they heard before.
The scientific method is intricately associated with science,
the process of human inquiry that pervades the modern era on many
levels. While the method appears simple and logical in description,
there is perhaps no more complex question than that of knowing
how we come to know things. In this introduction, we have emphasized
that the scientific method distinguishes science from other forms
of explanation because of its requirement of systematic experimentation.
We have also tried to point out some of the criteria and practices
developed by scientists to reduce the influence of individual
or social bias on scientific findings. Further investigations
of the scientific method and other aspects of scientific practice
may be found in the references listed below.
1. Wilson, E. Bright. An Introduction to Scientific Research (McGraw-Hill,
Kuhn, Thomas. The Structure of Scientific Revolutions (Univ. of
Chicago Press, 1962).
Barrow, John. Theories of Everything (Oxford Univ. Press, 1991).
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