Some of the advantages of a carefully planned animal study over a human study are obvious: the genetic makeup of laboratory mice and rats can be controlled.  They are specially bred for uniformity.  The environment and exposures also can be carefully controlled, so that cause and effect can be demonstrated more easily.  Generally, three or more different doses of a substance are used on different groups of rodents.  If a resulting cancer or other disease occurs at highest rates among the highest-dosed rodents, and less and less at the lower dosages, that reinforces a cause-and-effect relationship.  If a test rodent is known to handle a chemical in its body the way a human does, that also is important evidence.

In some of the newest studies, rodents are modified to carry human-like genes carrying increased susceptibility to diseases, such as cancer.

Skeptics may cry, "I'm a man (or woman), not a mouse!"  And they're right.  But genome studies have demonstrated that there is less difference than we might think.

And look at the test results: almost always, compounds that have proved to cause cancer in humans also have caused cancer in test animals.

While there are exceptions and some animals may be more sensitive to a substance than a human is, a carefully done animal study can often tell a lot.  Vinyl chloride was discovered to cause cancer in animal studies before such effects were observed in factory workers in Louisville, Ky.  Aflatoxin, which is a product of a fungus that can grow on peanuts, corn and other grains, was first observed to cause cancer in trout, then in lab animals and finally in epidemiological studies of human populations.

Sometimes the result of an animal study is a bit off, much as weather forecasts sometimes miss, but we live in an imperfect world where we prefer a weather forecast with its occasional false alarm, to no forecast at all.