3 Answers

Scientific control allows for comparisons of concepts. It is a part of the scientific method. Scientific control is often used in discussion of natural experiments. For instance, during drug testing, scientists will try to control two groups to keep them as identical and normal as possible, then allow one group to try the drug. Another example might be testing plant fertilizer by giving it to only half the plants in a garden (the plants that receive no fertilizer are the control group, because they are kept normal).

Scientific control needs not be experimental, and experimentation can sometimes be impossible (as in astronomy). The important thing is to try and control variables and attributes in the data so that the conclusions drawn are valid. Controls are used to try and avoid confounding variables, although this can be extremely difficult. In plainer words, scientific controls allow an investigator to make a claim like "Two situations were identical until factor X occurred, and so the new outcome was caused by factor X."   from:  http://en.wikipedia.org/wiki/Scientific_control

"To verify or regulate by conducting a parallel experiment."

CONTROL, as in control group, is what came to mind when I saw your question.  The CONTROL group would be the one that has no changes in the daily routine.  Say you are testing a new diet pill.  
One group would get the actual pill.  A second group would get a placebo.  Your group would get none; you would continue as you always do.

The control group gives you a base for comparison.  The placebo group probably has a lot of psychological stuff going on, thinking they are taking a diet pill.  Some will get hyper, others will lose weight, some may gain.....  The "real" group is the one whose responses are most valuable.   It give you a starting point.