The basic rationale for multivariate analysis is to allow the researcher to discount the alternative explanations of a relationship that can arise when a survey/correlational design has been employed. The experimental researcher can discount alternative explanations of a relationship through the combination of having a control group as well as an experimental group (or through a number of experimental groups) and random assignment (see Chapter 1). The absence of these characteristics, which in large part derives from the failure or inability to manipulate the independent variable in a survey/correlational study, means that a number of potentially confounding factors may exist. For example, we may find a relationship between people’s self-assigned social class (whether they describe themselves as middle or working class) and their voting preference (Conservative or Labour). But there are a number of problems that can be identified with interpreting such a relationship as causal. Could the relationship be spurious? This possibility could arise because people of higher incomes are both more likely to consider themselves middle class and to vote Conservative. Also, even if the relationship is not spurious, does the relationship apply equally to young and old? We know that age affects voting preferences, so how does this variable interact with self-assigned social class in regard to voting behaviour? Such a finding would imply that the class-voting relationship is moderated by age. The problem of spuriousness arises because we cannot make some people think they are middle class and others working class and then randomly assign subjects to the two categories. If we wanted to establish whether a moderated relationship exists whereby age moderated the class-voting relationship with an experimental study, we would use a factorial design (see Chapter 9). Obviously, we are not able to create such experimental conditions, so when we investigate this kind of issue through surveys, we have to recognise the limitations of inferring causal relationships from our data. In each of the two
questions about the class-voting relationship, a third variable – income and age respectively – potentially contaminates the relationship and forces us to be sceptical about it.