4. Public

The Observability of a product "has a huge impact on whether products and ideas catch on," says the book's author. A product that can be seen in the public eye is that "every time people use the product or service, they also transmit social proof or passive approval because usage is observable" a product's usage by others thus helps advertise it to others.

Making something more observable makes it easier to imitate. Thus a key factor in driving products to catch on is public visibility.