[Tom]

There are some patterns, one with every 4 series.

BB1 & BB5- Both original, trying something new, both a success
BB2 & BB6- Both expanding on the ideas from the previous year and being a moderate success
BB3 & BB7- Both regarded as two of the better series along with BB5. Both try something new and are very successful. Both are also examples of the 'institutional' Big Brother
BB4 & BB8- Both flops

Now if the 4 year gap thing continues, then BB9 will join onto that of BB1 and BB5 and if rumours are to believe, that is about right. Odd series tend to do better than even series.