Chapter Summary
When psychologists discover a relationship between two variables, there are a number of possible interpretations. Most importantly, they cannot be sure that a third variable is not the source of that relationship unless they conduct an experiment. Experiments allow for the elimination of alternative explanations for research findings. In order to conduct an experiment, a researcher must manipulate the independent variable, hold constant all other variables, and measure the outcome on a second, dependent variable. This allows them to determine that changes in the independent variable directly cause changes in the dependent variable.
The independent variable oftentimes has varying levels. For example, looking at the relationship between heat and aggression, researchers might manipulate the temperature of a room with different levels (e.g., 20 degrees versus 30 degrees). There would then be two different experimental groups of people: those who participated in the study a room-temperature environment and those who participated in a hot environment.
Another way to introduce different groups within the study is by having a control condition. Control groups serve as a sort of baseline measure; they represent normal, everyday responses in the absence of intervention by an experimenter. Placebo groups are related to control groups. Placebo groups receive “treatment,” but this is a benign or ineffective version of the treatment provided to the experimental group.
In addition to controlling for alternative explanations for a relationship between two variables, the power of experiments is that they allow researchers to eliminate causal directions. If X is manipulated and produces a change in Y, they can say with some certainty that X causes Y, rather than Y causing X. Although this is highly beneficial, it is also important to note that experiments are not always possible or appropriate.
In independent sample designs participants in the study experience only one condition of the study. This can create concern around non-equivalent groups, a situation where relevant personal characteristics may be disproportionately represented in one of the conditions, creating an experimental confound. In order to ensure equivalent groups, researchers rely on strategies such as basic random allocation, pre-testing, and representative allocation.
Alternatively, repeated measures designs deal with the problem of non-equivalence by exposing all participants to all the conditions of the study. However, this strategy has its own shortcomings, such as concern over effects related to the order that the conditions are presented. These order effects can be addressed by counterbalancing the order of materials and/or allowing sufficient time between presentation of the different conditions.
Finally, matched pairs designs attempt to create equivalent groups by matching participants along some meaningful criterion. Then, one participant is assigned to one condition, while the other participant is assigned to the opposite condition. This ensures that existing differences along this criterion are equally represented in the conditions of our study.
Sometimes, small n designs are warranted in order to answer our research questions. These studies may have a handful of participants, or possible even just one. Oftentimes this research is conducted with very unique individuals, where a larger sample is not possible, as is the case with those suffering from rare psychological conditions.
Up to this point, discussion of psychological research has focused predominantly on studies with a single independent variable. However, researchers often time include more than one independent variable in their studies. These independent variables are referred to as factors, and studies with multiple independent variables are called multi-factorial designs. These studies allow us to look at the simultaneous impact of different independent variables on the dependent variable.
Additional Online Resources
Online video and accompanying quiz on independent and dependent variables by Nathan Lampson: http://www.sophia.org/tutorials/independentdependent-variables--3
“Is this causal claim justified?”: http://www.everydayresearchmethods.com/current-affairs/
Links to various public press articles that claim causality—or do they? http://jfmueller.faculty.noctrl.edu/100/correlation_or_causation.htm
Flashcards
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