Some brands make it big; most do not. But why are certain brands successful in the first place? And why does a still-smaller group of brands remain popular over the long term?

In his 2010 book Brand Relevance: Making Competitors Irrelevant, US academic David A. Aaker attempts to answer these questions. He claims that "relevance" is a crucial factor in a brand attaining market leadership - and that companies become relevant by creating their own category or subcategory through innovation. Aaker presented his theory at a briefing organised by Prophet Brand Strategy in London earlier this week - and also agreed to talk to Warc about it.

To Aaker, advertisers should spend more on brand relevance initiatives and less on brand preference initiatives - in other words, competing against similar brands in established subcategories. "In brand preference, you win when the competitor brand is not preferred," Aaker said. "In brand relevance, you win when the competitor brand is not even considered."

David A. Aaker on brand relevance and innovation (1:19)

Sound interesting? We've put together a full Warc report on the brand relevance model, with all the key case studies discussed by Aaker at the event. These include the Kirin vs Asahi "beer wars" in Japan, Apple's iPod and iPad, and marketing initiatives from Walmart and Avon.

Aaker also maintains a - brand relevance focussed - blog. You can find out more about the book here.