From a model-building perspective, the 4 Ps has attracted a number of criticisms. Well-designed models should exhibit clearly defined categories that are mutually exclusive, with no overlap. Yet, the 4 Ps model has extensive overlapping problems. Several authors stress the hybrid nature of the fourth P, mentioning the presence of two important dimensions, "communication" (general and informative communications such as public relations and corporate communications) and "promotion" (persuasive communications such as advertising and direct selling). Certain marketing activities, such as personal selling, may be classified as either "promotion" or as part of the place (i.e., distribution) element. Some pricing tactics, such as promotional pricing, can be classified as price variables or promotional variables and, therefore, also exhibit some overlap.