Jay Tanzman wrote:
>I agree that you can test a hypothesis by using an observational study, but
>that
>does not make it an experiment. The original poster was looking for a
>definition to use in a lecture, and an experiment, by definition, involes
>assignment of treatments to experimental units.
>
>A study is hypothesis testing if the investigator is using it as such.
>Whether
>the particular study design would be expected to yield a valid answer is
>another
>matter.
In ecology, Hurlbert (1984) distinguished between manipulative and mensurative
experiments, "A manipulative experiment always involves two or more treatments.
The defining feature of a manipulative experiment is that the different
experimental units receive different treatments and that the assignment of
treatments to experimental units is or can be randomized."
Underwood (1997, p. 16) argued, "The distinction between types of experiments
is a distraction. It does not matter whether the system is measured or
manipulated or manipulated and measured. Each is appropriate for different
circumstances and different models. What matters is that the experiment is
clearly related to the need to test a logically defined null hypothesis. The
experiment must then be done so that it preserves the logical structure and
allows a logical conclusion."
The restriction of the term "experiment" to studies in which treatments are
assigned to experimental units, would rule out as non-experiments, many of the
more famous "experiments" in science. For example, Mayo (1996) describes
Eddington's 1919 observations of the deflection of starlight during eclipses as
experimental tests of Newton's and Einstein's theories of gravity. In 1918,
Eddington set out predictions which served as a crucial "experiment" to test
the predictions of Einstein and Newton. The deflection of starlight near the
sun during an eclipse was in near agreement with Einstein's gravitational
theory (within experimental error). Eddington's test would fall under the
category of a mensurative experiment or observational study, not a true
manipulative experiment. It would fit Underwood & Mayo's broad definition of
experiment.
Ernst Mayr (1982), the noted evolutionary biologist, adopted the strict
definition of experiment, "As Pantin (1968: 17) has stated, 'In astronomy, in
geology, and in biology observation of natural events at chosen times and
places can sometimes provide information as wholly sufficient for a conclusion
to be drawn as that which can be obtained by experiment
...contrary to the claims of some physicists, the branches of science which
depend on the comparative method are not inferior. "
So, there is some justification for the restrictive definition of experiment,
but there is also justification for a broad definition of experiment which
includes well-designed tests such as Eddington's.
Kendall and Stuart (1961) praise Fisher's advocacy of random assignment of
treatments to experimental units in order to solve problems inherent in
experimental designs, but they do not make randomization part of their
definition of an experiment, "The distinction between the design of experiments
and the design of sample surveys is fairly clear-cut, and may be expressed by
saying that in surveys we make observations on a sample taken from a finite
population of individuals, whereas in experiments we make observations which
are in principle generated by a hypothetical infinite population, in exactly
the same way that the tosses of a coin are. Of course, we may sometimes
experiment on the members of a sample resulting from a survey, or even make a
sample survey of the results of an (extensive) experiment, but the essential
distinction between the two fields should be clear."
Gene Gallagher
References
Hurlbert, S. J. 1984. Pseudoreplication and the design of ecological field
experiments. Ecol. Monogr 54: 187-211.
Kendall, M. G. and A. Stuart. 1961. The Advanced Theory of Statistics. Hafner.
Mayo, D. G. 1996. Error and the growth of experimental knowledge. U. of
Chicago Press.
Mayr, E. 1982. The growth of biological thought, Belknap Press, Cambridge
Underwood, A. J. 1997. Experiments in ecology. Cambridge University Press.
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