… the Classical Experimental Design have to do with its artificial nature; external and construct validities are the primary concerns. The threats to external validity include:
interaction between testing situation and the experiment stimulus
interaction between the experiment stimulus and the subject
interaction between the experiment stimulus and history.
Often, it isn’t possible to adopt a classical experimental design because it may not be possible to assign subjects at random to different conditions or treatments. Some Pre-Experimental Design studies solve this issue by either omitting the use of control groups or pre-tests. For example, a one-group, post-test design has only one group, a treatment and a post-test. But, this design is flawed because it has no way of telling whether the treatment had any influence on the group as distinct from other untested factors. A one-group pre-test/post-test design uses one group, a pre-test, a treatment and a post-test. This design is better than the one-group, post-test design because respondents are compared against themselves. But, because it lacks a control group, it leaves open the possibility that changes in the dependent variable might be a consequence of something …