Types of experiments: Natural and manipulative

Experiments in environmental toxicology can sometimes be manipulative experiments in which the researcher actively chooses and manipulates the independent variable. In Hunt's study, for example, dosages of BPA were manipulated and the effects were measured. In manipulative studies, the researcher controls all the other variables in the experiment, so any health effects observed in the test subjects can be attributed to differences in the independent variable.In other cases, researchers use natural experiments in which the dependent variable (typically a measure of organism health) is measured under differing contexts that are not manipulated. Say, for example, that an accidental chemical spill contaminates five ponds. To determine the possible effects of the toxic chemical on frogs, a researcher could compare the hatching rate of frog eggs laid in those five ponds to the hatching rate of eggs laid in five uncontaminated ponds nearby. This would be an example of a natural experiment because concentrations of the toxic chemical in the ponds were not controlled by the experimenter, but rather resulted from the chemical spill.

1. ____Blood concentrations of BPA in college students are compared to their recent consumption of canned food items
2. _the feeding behavior of fish in streams that receive acidic runoff from strip mines is compared to the feeding behavior of fish in unaffected streams.
3._the deformity rate in baby birds from nests in pesticide-sprayed fields is compared to the deformity rate in birds from nests in unsprayed fields
4 Tumor development is compared in mice exposed to five dosages of a known carcinogen in the laboratory
5. Foraging activity levels are compared in tadpoles exposed to four concentrations of toxic metals in the laboratory.
6.Growth of corn plants is compared in field plots sprayed with three different dosage:s of weed killer 7.______ BPA concentrations in the urine of people with diabetes are compared to BPA concentrations in the urine of people without diabetes

In a natural experiment, the researcher can specify that any proposed health implications could be due to exposure to the toxic chemical, but proving its direct causation is difficult. Natural experiments are majorly fixed to promoting correlation, unlike the manipulative experiments which usually strongly reveals causation.

Manipulative experiments changes the amounts of a predictor factor and estimates how one or more variables acts to these changes. It is employed in a test cause-and-effect. It cannot be used for large experimental scales as they are for small spatial scales experiments.

Natural experiments makes use of an observational study on natural variation in the variable of interest. It is used for large spatial and temporal scales without the need for manipulations but cannot be used for the direction of cause-and-effect.