It depends on which science. In social, behavioral, industrial, and many health related fields, the distinction is sharply drawn between true experiments where there is active manipulation of one or more treatment independent variables and random assignment of cases to treatment. (also, it simplifies calculation and drawing conclusions if there are equal n's in cells of the design.)
Other designs are considered quasi-experimental where plausible rival hypotheses need to be addressed by considerations other than manipulation and random assignment. The fewer aspects of a true experiment a study has the more discussion there needs to be of ruling out the rival hypotheses. The study you briefly describe would be called quasi-experimental. Some fields in statistics talk about what other fields would consider "thought experiments" such as ball-and-urns as experiments. The term "observation" has a wide variety of meanings, but some would include several kinds of quasi-experimental designs as observational - - - in your quasi-experiment you can possibly contrast different levels of specific pollutants, as well as kinds of pollutants, in different rivers at different times. I'm not a biologist, but I would be amazed if temperature did not affect population sizes. . Voltolini wrote: > Hi, > > I was reading a definition of "experiment" in science to be used in a > lecture and the use of treatments and controls are an important feature of > an experiment but.... my doubt is... is it possible to plan an experiment > without a control and call this as an "experiment" ? > > ================================================================= ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =================================================================