To test the effect of vitamin d on growth, two groups of rats were raised under identical conditions and fed the same diet. one of the groups received daily injections of vitamin d. the other group received injections of saline, which did not contain vitamin d. all the rats were weighed weekly for 2 months. in this experiment, the control was the

In this experiment, the control was the group of rats that received an injection of saline, instead of vitamin D. When conducting scientific experiments testing the effect of certain variables on a condition or a process, the experimental design needs to include a control group. Any observed differences between the control and the experimental group will be attributed to the independent variable of the experiment, in our example the vitamin D.
More specifically, in this example, the control group lived under the same conditions as the experimental group and received the same treatment (daily injections) but the only difference was that they were given saline instead of vitamin D. So, by using the control group, researchers made sure that any differences observed in the growth rate of these two groups of rats, were not due to experimental errors or bias.