Full document / COSOC-W-86-002 - the National Sea Grant Library

Independent variable that is assumed to be causally prior to it.
Manipulation occurs when the experimenter Intentionally altera the
Independent variable. Usually there are a number of confounding
variables and rival explanations which one attempts to eliminate
through a control. Then, if repeated manipulations of the
independent variable produce changes in the dependent variable,
one concludes the Independent variable had an effect.
Social programs such as pollution control are
quasl-experlments (Roos and Bonner, 1973). One variable is
manipulated through the apparatus of government with the hope that
the desired change will ensue. The evaluation of results is
difficult because these experiments contain some but not all
elements of of the ideal natural science experiment. As a result
the task for the analyst is to design an approach that compensates
for these weaknesses and hence leads to valid causal Inferences.
The challenge is to use the logic of experimentation in situations
that do not meet all the requirements of an ideal experiment.
Primary among tha obstacles in a quasl-experlment is the faot that
the random assignment of units and the control of treatments are
beyond the grasp of the investigator. Information concerning
events that have occurred must be extracted from records that were
not necessarily kept to test the hypotheses of lntorest. The
method emphasizes improvements In design over advanced data
analysis as a means to infer cause.
For a reasonable chance of success the quasl-experlment must
Include several elements of logic that parallel those of an ideal
laboratory experiment (Caparaso, 1973). Plrst, the independent
variable must shift enough through a known manipulation to produce
a reasonable expectation of a change in the dependent variable.
Seoond, the effect of the change of tho •independent variable on
the dependent variable must go beyond that produced by chance
alone. Third, plausible rival hypotheses must be examined and
ruled out to support the case for a causal relationship.
Estuarine Pollution Control as A Ouasl-Excerlment
Governmental goals for water quality and expenditures for
instruments, the sewage treatment plants, are tho basis of the
hypothesis that sewage treatment plants reduce estuarine water
pollution. The Independent variable is the Initiation of plant
functioning and the dependent variables are amounts of pollutants
in the estuary. Testing the hypothesis requires the assembly of
pertinent, accurate data on water quality in the estuary and
possible influences on it including and beyond the cause
hypothesized. The latter can be important because the strategy of
a quasl-experlment is to identify correlation and to distinguish
correlation froi" causation (Hoole, 1978). The approach requires
the systematic testing of rival explanations before Judgements
about cause may be rendered. Ultimately the acceptance of the
hypothesized cause is dependent upon careful analysis of the
individual situation. The cause must precede the effect in time.
Treatments must covary with effects. There must be no more
attractive alternative explanations.
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