what is the purpose for a control in scientific experiments?

Answer 1

Control is a sample that you use to check whether the experiment has worked the way you intended from a "technical" point of view - i.e whether the results you are looking at, are really due to biological/chemical effects and not because of experimental problems.

You need to distinguish between positive control and negative control. A negative control is, generally, a reaction where something has been omitted and you should have no signal. That rules out that, if your results are positive, you are not looking at false positives! For example, if you want to check the effect of a drug on cells (say, the S-phase inhibitor aphidicolin), than you'll leave a sample of cells untreated as comparison.
A positive control is a sample treated in such a way that you can predict the result - normally a strong positive signal. This will tell you that, if you DO NOT see this signal, the whole experiment did not work and you can scrap it. In the previous example, you could use Hydroxyurea, which is known to block S-phase in mammalian cells.

Answer 2

the purpose of control in a scientific experiment is that you want to have something that's consistent through the whole experiment. the control stays the same through the whole experiment and never changes. the control is the subject in the experiment that the person conducting the experiment keeps the same all the way through and it isn't munipulated in any way.

Answer 3

The control is there to observe any changes in the item being studied to isolate any changes that might be attributed to the test conditions you hope to measure.

In medical tests for example, 100 people might be given a 'dummy' drug or 'placebo' which will do nothing for them.

Another 100 will be given the real thing.

Out of the first 100, maybe (say) 10 will become well anyway.

If only 10 in the other group get well then its back to the drawing board.

Without the control group, the recovery of the 10 in the 'live' tests would have been attributed to the new drug.

Answer 4

To compare the unknown effect of the experiement against a known effect of a reference source. How do you know you are sick with a fever if you don't know what your temperature is when your well?

Answer 5

it purpose is to avoid the so-called confounding factors like (age, ability, educational level etc.), which affect the relationship between the two variables.
for example if you are examining the relationship between educational level and income in a particular group of people, you would like this group to be as homogeneous as possible. the confounding factors, like background, ambition and ability may affect the result of your study significantly.

Answer 6

to have something to compare to