What this node touches on is a fundamental dichotomy that exists in the scientific method, as is practised today. In science, there are generally two ways of examining a system of interest: by way of mensurative or manipulative investigation. The conclusions that can be drawn from the study (and the statistical methods available to the researchers) vary depending upon which approach is taken.

What leighton describes as a "proper scientific experiment" is in fact the manipulative approach to investigation. As he states, the researcher has considerable control over the subjects (they need not be patients or animals, in passing -- they can be any conceivable object) and can control the system of interest (read: control over independant variables). The advantage to this type of approach is that the results, if they are significant, can be readily attributed to the manipulation. Thus, if treat one group of patients pre-disposed to heart conditions with aspirin and give the others a placebo, the observed effect can be attributed to our manipulation of the system (dosing with AAS). In general, in these types of experiments, the experimental design and subsequent analysis of the data follow the model of an ANOVA.

What leighton describes as a pseudoexperiment is in fact a mensurative experiment, wherein the researcher does not manipulate the system of interest, but instead collects information about the system and attempts to control (statistically) for confounding variables in order to determine whether or not a hypothesized dynamic exists. This is not only a perfectly acceptable method of scientific inquiry, but often is the only one that is available in many fields. For example, epidemiologists, astronomers and ecologists often have no option but to simply observe the system of interest and attempt to draw conclusions from the patterns within the data. Experimental design, in a mensurative experiment, is considerably more complex (at times), and the statistical methods employed are more likely to be correlative or predictive (regression) in nature.

I thought that this needed to be added, given the pejorative nature of the term pseudoexperiment. Many scientists spend their lives and careers working very hard uncovering truths without evern performing a manipulative experiment