Science succeeds or fails in its efforts to produce valuable technology and policy based upon the quality of its theories. When theory is incorrectly formulated or hypotheses are incorrectly deduced from theory, then technology and social policy fail.

Unfortunately, it is very easy to build incorrect theory or incorrectly deduce from theory. And all too often, mistakes in theory induction or deduction are not revealed until people experience the negative consequences of flawed technology or social policy.

Mistakes in theory induction and deduction can be summarized as two types of false relationships between variables: spurious relationships and suppressor relationships.

Spurious Relationships

Spurious relationships are false indications of causality. They occur when theory omits an extraneous variable that affects two other variables that have no causal connection. The effects of the extraneous variable on the two other variables leads one to conclude, in error, that the two other variables are causally linked.

This silly example illustrates a spurious relationship. City Manager Sapp, of Sappville, has collected data on the past 100 fires to occur in the city. He has information on the number of fire trucks dispatched to each fire and the amount of damage (in dollars) caused by these fires. Mr. Sapp notices a very strong relationship between the number of fire trucks dispatched and the amount of fire damage. He concludes, logically, from the data at hand, that "if we stop sending fire trucks to fires we will greatly reduce fire damage in Sappville." What Mr. Sapp did not realize from his limited data was that a third variable--size of fire--caused variation in both of the events he recorded: number of fire trucks dispatched and amount of fire damage.

This spurious relationship, or false indication of causality, can be diagramed as such:

diagram of a spurious relationship
[D]

The false indication of causality is the relationship between number of fire trucks dispatched and amount of fire damage. Size of fire is the extraneous, or unrecognized, variable.

Sound silly? Think it unlikely one would make such a mistake? Consider this theoretical proposition: The greater the school expenditures per pupil, the greater the academic performance. Sounds reasonable; but this relationship is a false indication of causality. The extraneous variable is parental encouragement to achieve. Parents with higher levels of education tend to have higher incomes and live in areas that spend more per pupil on education. Children of parents with higher levels of education tend to achieve academically. But spending more per pupil does not lead to higher academic performance in areas where parents do not motivate their children to perform academically.

Said simply: Correlation does not mean causation.


Application in Context

    Spurious Effects on World Hunger?
    This comment on world hunger argues that the efforts of the Gates Foundation to ease world hunger actually will increase hunger because the "technology-driven" policies of the Foundation ignore the effects of extraneous variables.

Suppressor Relationships

Suppressor relationships are false indications of no causality. In this case, the extraneous variable has a positive relationship with one variable and a negative relationship with another variable. Although these two other variables are causally connected, it seems like there is no relationship between them because the extraneous variable "cancels out" the correlation between them.

Consider this theoretical proposition: The greater the academic performance, the greater the job performance. For challenging jobs, there is a causal connection between academic performance and job performance. For routine jobs or those involving much repetition, however, there is little correlation between academic performance and job performance. The extraneous variable is boredom with the job. For routine or repetitive jobs, persons who excel academically have poor job performance because they tend to get bored.

This suppressor relationship, or false indication of no causality, can be diagramed as such:

diagram of a suppressor relationship
[D]

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