goes up. In addition, the increase in product variety reduces the impact of user reviews on popular products more than it does on niche products. Networks, crowds, and the long tail The "crowds" of customers, users and small companies that inhabit the long-tail distribution can perform collaborative and assignment work. Some relevant forms of these new production models are: The peer-to-peer collaboration groups that produce open-source software or create wikis such as Wikipedia. The crowdsourcing model, in which a company outsources work to a large group of market players using a collaborative online platform. The model of crowdcasting, is the process of building a network of users and then delivering challenges or tasks to be solved with the purpose of gaining insights or innovative ideas. Work performed by individuals in commons-like, non-market networks, described in the work of Yochai Benkler. The demand-side factors that lead to the long tail can be amplified by the "networks of products" which are created by hyperlinked recommendations across products. An MIS Quarterly article by Gal Oestreicher-Singer and Arun Sundararajan shows that categories of books on Amazon.com which are more central and thus influenced more by their recommendation network have significantly more pronounced long-tail distributions. Their data across 200 subject areas shows that a doubling of this influence leads to a 50% increase in revenues from the least popular one-fifth of books. Turnover within the long tail The long-tail distribution applies at a given point in time, but over time the relative popularity of the sales of the individual products will change. Although the distribution of sales may appear to be similar over time, the positions of the individual items within it will vary. For example, new items constantly enter most fashion markets. A recent fashion-based model of consumer choice, which is capable of generating power law distributions of sales similar to those observed in practice, takes into account turnover in the relative sales of a given set of items, as well as innovation, in the sense that entirely new items become offered for sale. There may be an optimal inventory size, given the balance between sales and the cost of keeping up with the turnover. An analysis based on this pure fashion model indicates that, even for digital retailers, the optimal inventory may in many cases be less than the millions of items that they can potentially offer. In other words, by proceeding further