When does a hit single actually become a Number One?

Well, here’s an interesting one. I want to be able to make statements along the lines of “song / album X went to number one on date Y” for every single and album by the band I’m researching. I’ve been fortunate enough to get some extremely helpful feedback from the masters of this subject, Sir Tim Rice, Paul Gambaccini and Jo Rice, the compilers and authors of the original ‘Guinness Book of British Hit Singles’ and ‘Guinness Book of British Hit Albums’. Here’s the problem: the charts are compiled using sales data for a particular week (sales date); some time after the ‘sales date’ the new chart is first announced, usually on a radio show (announcement date); when the chart is announced it has an official ‘publication date’, which is usually later than the actual ‘announcement date’.

e.g.
‘Waterloo’ by Abba was first announced as the new UK number one on Tuesday 30th April 1974, on the BBC Radio 1 lunch-time show. The chart was compiled using sales data for the week ending Saturday 27th April, and had an official publication date of Saturday 4th May. The official publication date is listed in the Official Chart Company database and the Guinness Book of British Hit Singles, but is it really the date ‘Waterloo’ went to number one?

To confuse matters further, as Jo Rice pointed out to me, in the 1960s Beatles singles often sold more on the first day of a new week than their nearest rivals sold in the entire week. Based on this, you could argue that such a single ‘went to number one’ on the first day of the week which ended on the ‘sales date’.

Even now, in these days of high speed computing and virtually instant communication, the Official Chart company use an official publication date of the Saturday following the first announcement date (on the BBC Radio 1 Sunday Chart Show) and the chart is compiled using sales data from the week ending on the Saturday before the Sunday announcement date.

So, how do I solve this problem, bearing in mind that most official sources quote a chart date, which differs from the announcement date? I’ve decided to quote both, so every chart entry and highest position date will get two entries; “In the chart announced today, song X was confirmed as the new number one” and “In the chart published with today’s date, song X was listed as the new number one”. Phew, no wonder this is all taking a lot longer than I was expecting.