A variable can change so how it effect the exerinment varies. go to http://www.isd77.k12.mn.us/resources/cf/SciProjInter.html this website may help in answering your question.
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That will result in "replications" of the experiment.
If none of the variables are constant (or controls) you have no idea which variable or combination of variables caused the effect.
Endogenous variables are important in econometrics and economic modeling because they show whether a variable causes a particular effect. Economists employ causal modeling to explain outcomes (dependent variables) based on a variety of factors (independent variables), and to determine to which extent a result can be attributed to an endogenous or exogenous cause.
Dependent upon the variables, you need to take into consideration factors that can affect the outcome of the result; what will make the result vary in any way. If this, for example, entails the variable to be kept constant time, you will monitor the time and repeat it throughout the experiment. This is my understanding of constant variables; hope this helped.
In a controlled experiment, the Independent variable refers to the variable that is manipulated or altered. The dependent variable, meanwhile, is the result of the experiment.