A mensurative experiment is one in which the experimenter does not
manipulate the
system of study. In other words, the experimenter collects information about the system in its natural state, with all its inherent
variability. Generally speaking, experimenters performing mensurative experiments have to collect more
observations than their manipulating counterparts in order to
statistically control for the effects of those confounding
variables of little interest. Here are three examples of mensurative experiments:
- An excellent example of a series of mensurative studies is the Long Term Ecological Research LTER network, which is a collaborative effort by more than 1000 researchers with the expressed goal of intensively monitoring 24 sites (each representing a different ecosystem) over a long period of time. The amount of data collected by the project, to date, is astounding, but as an example let me cite the data gathered from the Parker River Plum Island Sound, MA. In this monitoring project, the researchers gathered information on the following variables each month at a number of sites over six years:
This large series of variables can be later fused with local meterological data, climate warming data, and hydrodynamic data in order to predict such things as primary productivity, fish production or perhaps even the productivity of the local fishing industry.
- A second example of a mensurative study was the examination into the link between electromagnetic fields and cancer (the high tension power line dispute) in 1997, conducted by Martha Linet of the National Cancer Institute and Leslie Robison of the University of Minnesota. They used, as their subjects, 638 children diagnosed with leukemia and 620 matched controls in the southeast of the United States. Their study controlled for the potentially confounding effects of extraneous variables (diet, family incidence, smoking etc.) and found no effect, contrary to public opinion.
- Perhaps the most famous example of a mensurative experiment is the series of investigations performed to determine the link between smoking and cancer (not to mention the other associated health concerns). The researchers obviously had no control over their subjects (they couldn't ask people to begin smoking, and to continue to do so for twenty years), and instead were obliged to merely compare the incidence of cancer between smokers and non-smokers. By collecting a tremendous amount of information about the people being studied (diet, work environment, home environment, family medical history, social status, locality etc.), they were able to tease apart the effects of smoking from the confounding variables, and determine that it indeed is associated with a greatly increased risk of cancer.
There are many further
examples of mensurative experiments, and the preceeding one shows that despite an absence of control over the system, conclusions can be reached.
I should add, as well, that there are examples of mensurative experiments in nearly every domain of scientific inquiry, and that I hope others will provide further examples in this node.