On Top40Weekly.com, for the weekly chart “US Top 40 Singles Week Ending 30th September, 1972”, the number of weeks on the chart for the No. 13 single that week (i.e. “My Ding-a-Ling” by Chuck Berry) shows 7 cumulative weeks. But if you check the chart for the previous week (“US Top 40 Singles Week Ending 23rd September, 1972”), this same single shows 8 cumulative weeks on the chart. This is obviously an error, and it means that every “weeks on chart” value for this single is off by two for the remainder of its chart life in 1972.