This is an attempt to define a DTD for an experiment design. As you can see, a DTD is a simple tree structure. In our case, this is an experiment. The target is really just the experiment design, but since at least one design engine already accommodates previously run trials, the structure will hold experiment results as well. There are both leaves and branches coming off the root.
Branches are the elements of the central object.
The leaves are Attributes, Comments, Text. Attribute Attributes are descriptive characteristics of an element. Attribute values can be required or optional. All are optional in this DTD for maximum flexibility, but obviously most have to be filled at one point in the process or another. In the tree above, all of the items that do not have sub items are attributes.