control animal
Meaning of control animal: The untreated half of an animal experiment, and the only reason a change in the treated half can be credited to anything.
Definition of control animal
A control animal is one kept in a study for comparison rather than to receive the intervention being tested. It is housed, handled, fed and assessed the same way as the treated animals, and often receives a dummy version of the treatment. Differences that then emerge between the two sets can be attributed to the intervention rather than to the conditions both groups shared.
Why is leaving an animal alone not good enough?
Because the act of treating an animal changes it, quite apart from whatever is given. Being caught, held and injected raises stress hormones, and stress alone shifts weight, immune function and behavior. If treated animals are handled daily while the comparison animals are left undisturbed, any difference found might be the handling. A proper control therefore receives everything except the active ingredient: the same injection, at the same time, from the same person, containing only the liquid the drug was dissolved in. That liquid is called the vehicle, and vehicle controls are the usual standard in any animal study.
What kinds are used?
| Type | What it receives | What it isolates |
|---|---|---|
| Untreated control | Nothing beyond routine care | The effect of the whole procedure |
| Vehicle or sham control | Everything but the active ingredient | The effect of the ingredient itself |
| Positive control | A treatment already known to work | Whether the experiment can detect anything |
| Littermate control | Shared parents and cage as well | The effect of background and breeding |
The positive control is the one most often omitted and the most revealing. When a treatment known to work fails to produce its usual result, the experiment itself is not functioning, and a negative finding for the new treatment means nothing at all.
What makes the comparison trustworthy?
The same safeguards used in human research. Animals are assigned to groups at random rather than by whichever is easiest to catch first, since the easiest to catch tend to be the slowest and least well. Whoever measures the outcome is kept unaware of which group each animal belonged to, because expectation shapes judgment when scoring behavior or reading a slide. Group sizes are fixed before the work begins rather than extended until a difference appears. Reporting checklists exist because these steps were often skipped and more often left undescribed, and studies missing them tend to overstate the effects they report. The logic matches that of a control group in a human trial, and the animals themselves form part of an animal model of the condition being studied.
Used in a sentence
Tumor growth in the treated group was compared with growth in the control animals.
Good to know
A comparison group simply left alone cannot separate the effect of a substance from the effect of being caught, handled and injected, which is why control animals usually receive the carrier liquid on the same schedule.