Experimental design refers to the framework or structure of an experiments and as such there are several experimental designs. We can classify experimental design into two broad categories viz informal experimental design and formal experimental designs.
When these designs are used many of the effects related to the experiment can be quantified. But because of the fairly loose way in which they are structured the changes specifically caused by the treatment cannot be isolated by statistical tests. The treatments are not allocated to experimental units randomly but these designs are less expensive and easy to apply in practice.