How do you formulate a Christmas number one?

It is one of the most coveted prizes in music, but how do you make sure you bag the Christmas number one spot?Experts at UK music label Ostereo think they have cracked the formula, after crunching the statistics on every Christmas number one from the last 50 years.Researchers found most number ones are ballads, the majority were covers, and nearly all were about something other than Christmas.