Pre-experimental design is a research format in which some basic experimental attributes are used while some are not. This factor causes an experiment to not qualify as truly experimental. This type of design is commonly used as a cost effective way to conduct exploratory research to see if there is any evidence that warrants a full scale experimental study (performed 'pre-experiment').
One type of pre-experimental design is the one shot case study in which one group is exposed to a treatment or condition and measured afterwards to see if there were any effects. There is no control group for comparison. An example of this would be a teacher using a new instructional method for their class. Their class could be measured at the end of the term to see if the new instructional method was effective. Another type is when one group is given a pretest, exposed to the treatment or condition, and then given a posttest to see if the treatment had any effect on the group.
For example, the teacher using the new instructional method gives their students a pretest at the beginning of the term, teaches the new method throughout the term, and then gives them a posttest at the end of the term. Again, there is no comparison group. Another type of pre-experimental design is a static group comparison. This is when two groups (one is given the treatment or condition, one isn't) are both given posttests to see if the treatment had any effects. Because no pretest is given it is unknown if there were any group differences before exposure to the treatment.
An example of this would be finding one class who had been exposed to the new instructional method and one class who had not been exposed and giving them both posttests at the end of the term. Pre-experimental designs are threatened by a lack of validity because it can be uncertain if any of the effects were due to the treatment or other hypotheses/explanations. That is why this type of design is best for exploratory purposes and in a truly 'pre-experiment' way.