In a biological experiment, there are several different variables that help a scientist discover new information. The independent variable is the aspect of the experiment that is changed or manipulated to find out an answer, while the dependent variable is the part of the experiment that is affected by the change in the independent variable. Standardised variables are those that are unaffected by the experiment.
The standardised variables in an experiment are always the same. For example, in an experiment determining whether or not age (an independent variable) has an effect on ease of weight loss (the dependent variable), all other aspects of the experiment other than age must be the same between groups. If there is a group of 25-year-old men and a group of 45-year-old men being tested, their diets, exercise programs and stress levels must be the same. Diet, exercise and stress are standardised variables -- the variable is kept the constant, or "standardised," for each group.
Allow Broad Application
When the standardised variables are kept constant for each independent variable, then the results of the experiments can be standardised across an entire population. If an experiment studies how well a certain seed grows in heavy rainfall versus light rainfall, then factors such as light, heat, planting depth and fertiliser must be standardised variables. If they are standardised, then the experimenter can say the results would apply anywhere these seeds are planted. Without standardisation, then the results can only apply in situations identical to the ones found in the experiment, and it is impossible to draw conclusions between the two groups or apply the results to other groups.
If the other variables are standardised, then an experimenter can comfortably say that the independent variable is actually having an effect. In an experiment comparing two different types of seeds, if one group of seeds gets watered twice as much as the other group of seeds, then an experimenter has no idea if the independent variable (the type of seed) affected the results, or if it was the difference in the amount of water the seeds received that effected the change. By standardising the variable of water -- that is, keeping it the same -- the experiment can show that the independent variable caused the effects shown in the experiment.
In an experiment determining if a new drug lowers cholesterol levels more than a placebo or more than another drug, the independent variable is the different drugs and the placebo. The dependent variable is the level of cholesterol, and the standardised variables are the age of the subjects, the relative health of the subjects, the additives or fillers in the drugs/placebo, the frequency of the drug administration and the frequency with which the cholesterol levels are checked.