Internal Validity Defintion and
                    External Validity Definition

  It is important to evaluate the validity of experiments.  
Two type of validity are internal validity and external
validity.  What is internal validity?  What is external
validity?  With respect to an experiment, below is a
definition of internal validity and a definition of external

Internal Validity Definition:

An experiment has internal validity if the effect on the
dependent variable can be clearly attributed to the
independent variable.

External Validity Definition:

The external validity of an experiment reflects the extent
to which the findings of the experiment would generalize
to other participants or situations.