Bob G says in this morning’s Observer (can’t find the link, sorry) that in order to raise the most awareness for African debt relief he needs the biggest selling artists, hence African musicians are being left off. “It’s not a cultural event”, apparently, so only the brightest stars – 15 million albums and rising – need apply. It seems to make sense, but really it doesn’t – even if Bob’s startling info that Velvet Revolver have sold 30 million records is correct.

Because if it isn’t a cultural event, and maximum audience is the name of the game, Bob needs to look not just at the sales figures but at the audience reach of his line-up. In marketing we call this range optimisation.

Say you’re a supermarket, with room for four cola brands on your shelves. You could just pick the top four colas in the market and be assured of maximum sales, right? Wrong. If the four top colas are all full-sugar and the fifth cola is a diet one, then it’s likely you’ll reach a wider section of the market by picking 3 out of the top 4 and number 5: the audience for the diet cola is going to include unique individuals who won’t buy any of the other four, which are in some sense interchangeable.

This becomes even more important when you’re dealing with brands – like pop acts – whose consumer base is likely to be loyal to several different acts. Putting together any gig or festival line-up is a matter of balancing the pleasure of the audience (who in general will want to see similar things that they like) with the potential size of the audience (more diversity luring more people).

Usually the limited venue capacity means that you should ignore diversity and concentrate on increasing consumer satisfaction. When I went to see Girls Aloud last week, for instance, they weren’t on a double-header with My Chemical Romance, a bill which would have increased the potential audience but left many fans of each group feeling ripped-off by the presence of the other.

But Live8 isn’t like this: the stated aim is to get maximum awareness, i.e. the highest possible number of people watching on TV worldwide. So audience satisfaction is less of a priority than maximising the audience reach through diversity. Normal promoter and gig logic does not apply.

In other words, the question Bob needs to ask is this: if you already have Coldplay and Keane on the bill, how many extra people will you reach by putting Snow Patrol on, and is it more or less than you’d reach by putting Baaba Maal on?