A pattern language is effectively constructed when the confidence of the collected patterns is assessed, and especially when interrelated patterns are cross-referenced. Technically, this happens during steps 5 and 6 in the process described in the earlier post How Patterns become a Pattern Language . While these steps are conceptually simple activities, they are the constituents of the true expressive power and notational richness of the pattern language. There are many aspects that must be taken into consideration and which are summarized hereafter.
Just collecting a number of patterns alone is not sufficient to define the pattern language. The quality of the pattern to be used must be recognized. One aspect is its recurrence in reality. A pattern that is discovered simultaneously in different organizations, is certainly more reliable than one that has been observed rarely. The higher the number of instance, the greater the confidence. The confidence is a judgment of the pattern’s invariance ([LOWE-2006]), i.e. of its immutability. It relates directly to the original formulation that patterns should be timeless ([ALEXANDER-1979]).
Another dimension is the impact of the effect of a pattern. Some patterns might not recur so frequently, but their impact is obvious and evident to the people on which it acts. In other words, consistent reports of good outcomes should also affect the confidence assessment ([LOWE-2006]).
The impact and effects of a pattern might also be recognized as negative; they diminish the desired outcomes. In extreme cases the forces involved are of such strength that they cause negative consequences leading to failure or breakdown. Such patterns are known as antipatterns ([KOENIG-1995]; [LOWE-2006]).
There are also neutral patterns, which do not present workable solutions, but are just a description of context and forces; nonetheless they might be valuable in gaining an understanding of a particular situation, or for establishing the initial conditions. Often neutral patterns represent constraints which are outside of human control. All these kinds of patterns (positive, neutral and anti-patterns) should be considered as part of the lexicon ([LOWE-2006]).
One of the most important activities, in establishing a pattern language, is recognizing the interconnections between the patterns. It is the network of interconnections that truly constitutes the pattern language ([ALEXANDER-1977]). Patterns exist in a hierarchy: any pattern might contribute to the solution of higher-level patterns, while it is itself the context for lower-level patterns ([LOWE-2006]).
It is the network of interconnections that turns the catalog into a body of knowledge ([SCHULER-2008]); thus the quality of the interconnections are of essence. The interconnections are the connective rules that determine how the patterns can be combined.
The rules that are used to connect the patterns, are just as important as the patterns themselves ([SALINGAROS-2000]). The pattern catalog alone is insufficient to convey this properly; the representation of a connective map necessary ([SALINGAROS-2000]).