When you are doing experiments in science, no matter what type, there are things that you manipulate, and results that occur as a result. The things you are in charge of are independent variables (IV). You may decide to test plant growth at different temperatures, or different carbon dioxide levels. The temp or CO2 level is your independent variable--you decide what level. The amount the plants grow is your dependent variable(DV)--the amount will be different, depending on your independent variable. When you graph your result, the IV goes on the X axis, the DV on the Y.
Extraneous variables are things that influence your results, and are a source of error. These are the things we attempt to control. For instance, if you are testing the influence of temp on plant growth, you want to make sure that soil type and amount of water given are constant; otherwise, they are extraneous variables.
In research, a variable is defined as any factor that can be manipulated and measured. In a research variables take on different values which are observed and measured to draw conclusion about the subject under study.
Variables are grouped in many different types and according to their nature. One of the ways of classifying the variable is depending upon the way these take on different values during a research or experiment and the nature of influence they have on the research outcomes. As per this classification variables are classified as dependent, independent and extraneous variables.
Independent variables are isolated and controlled by the researcher during the experiment or the research study. The dependent variable changes as a result of, or in response to, the variations in the dependent variable. Extraneous variable are neither manipulated by the researcher nor change in response to changes in independent variables, but which may have some influence on the outcome of an experiment including the values of the dependent variables. Therefore researchers try to ensure that there is no changes in the values taken by the extraneous variable during an experiment.
To understand these term better let us take the example of impact of a particular nutrient, say calcium, on increase in weight of rats. A researcher can study this by feeding different quantities of calcium to different rats and observing the increase in their weight. In this case the quantity of calcium in the diet of a rat is the independent variable, and the increase in weight of the mouse is dependent variable. However, increase in weight of rats also depends on other variables such as other nutrients in their diet, their original weight, and living conditions of mice. These are the extraneous variable. The researcher can eliminate the impact of these extraneous variable by ensuring that all the rats used in the experiment have comparable age and weight at the start of experiment and live under similar condition during the experiment. Also he or she will ensure that diet fed to all the rats is identical except for the quantity of calcium.
When performing a study of some event, the independent variable is the thing that is controled. For example, if you are studying the effects of an inspirational video on freshmen undergrads. So we divide this population into two groups. The independent variable is whether or not a group watched the video or did not watch the video.
The dependent variable is the item you are measuring to see if there was a change. In the example above, the dependent variable could be the test scores of both groups (ideally, both groups are taking the same class and professor and therefore the same test).
Extraneous variables are any variables outside of those above. Many times it is a variable that the researcher did not plan on that influenced the outcome of the study. In our example, and extraneous variable might that one of the students in the group hard of hearing and therefore did not get the full impact of the movie. Another example might be that one of the students was sick on the day one of the tests was given and therefore did poorly on the exam. Any study will try to minimize these variables but it is impossible to handle everything.