We are pleased to announce the launch of the Product Assortment Index. The index will track the size and shape of the product assortments offered by 11 of the largest online retailers and marketplaces in North America. The 11 members of the index generated over $84 billion in revenue in 2013, which represents 53% of the revenue generated by the Internet Retailer Top 40. These are the leading retailers and marketplaces in North America. These retailers and marketplaces offered over 350 million  listed products as of March 14, 2014.
Here’s how the Product Assortment Index works (PAI). Every week, we visit the websites of the 11 members and count the number of products available in each category. This data is then aggregated and summarized in an easy to consume format that provides a barometer of the overall growth in product assortments online.
For the week ending March 14th the PAI was down 5% from the beginning of 2014, which represented a loss of 16 million product listings across the 11 retailers and marketplaces since January 1st. However, growth was unevenly split between 1st-Party Retail and Marketplaces. Since January 1st, Marketplaces have seen a decline of 5% (a loss of 16 million product listings), while 1st-Party Retail assortments have grown 18% — a gain of over 2 million product listings.
The general trend we’ve observed is that Marketplaces are significantly more aggressive at both listing new products and de-listing products.
Since mid-December, Marketplaces represent 94% of the net-new product listings across the 11 retail/marketplace basket.
The above graph shows the PAI since mid-December, split between Marketplace and Retail. The greater variability of the Marketplace assortments is clearly evident. First party retail assortments have tended to increase steadily, while the marketplace assortments have a high degree of variability.
That’s all for the first blog post on the new Product Assortment Index. Let us know what you think!
 The PAI tracks product listings, which we define as the non-unique offer of a single product by a retailer in a category. If a product is listed on a retailer’s website in four categories, we count that as four product listings. We do not count product variations — a shoe that comes in 10 sizes and 3 colors is counted as 1 product listing.