Virtually all scientific experiments are done using the same logic: an experimenter changes or varies one factor and observes or measures what happens as a result of the manipulation.


Three types of variables:

1. The factor that you vary in order to answer your question is known as the independent variable

2. The factor that is measured or observed is known as the dependent variable.

3. At the same time, the experimenter is probably making a special effort to keep some factors    

    unvaried over all the trials so that they can't affect the outcome.

    Those factors are control variables.


Independent vs. Dependent variable    


Question:  "How do changes in temperature affect the reactivity of an enzyme"

  You are measuring the enzymeís activity at different temperatures.

  TEMPERATURE is the independent variable.    

  You decided to vary the temperatures to see how this effects the enzyme.  It is the variable

  determined by you.

  You cannot control how this will effect the reactivity of the enzyme. 

  REACTIVITY is the dependent variable.  It is determined by the amount of substrate that remains 

  at each temperature.  Therefore, the amount of reactivity of the enzyme depends on the

  temperature.  Thatís why the time is the dependent variable..


Treatments of the Independent Variable

The independent variable is controlled by the experimenter, who splits the subjects into different groups, with each group being associated with a different treatment (or value) of the independent variable. The treatment can simply be to identify what group a subject falls in, or it can be associated with a numeric value.



Problem:  How does time effect the reactivity of an enzyme

The Independent variable:  the time intervals

The Treatment:  the intervals of your choosing, for example 60 seconds.

The Dependent variable: the amount of substrate remaining after each 60 second interval, which is what you will be measuring.


The experimenter controls the independent variable, the TIME intervals.

The independent variable controls the dependent variable, the REACTIVITY of the enzyme.

The dependent variable depends on the treatments (different conditions) applied to the independent