Casey Kasem and his staff used R&R for the source of their charts from January 1989 to October 14, 2000. On October 21, 2000, AT40 switched from R&R to a chart provided by Mediabase 24/7. The Mediabase 24/7 chart appeared to have no coherent recurrent rule, and songs were removed from the chart from as high as #10. AT40 said that their new chart was better than R&R's, and that the new chart would provide a "fresher" sound since the chart supposedly moved faster. R&R staff have said that AT40 actually changed charts because Mediabase charts were available to AT40 a few days earlier, giving AT40 writers more time to prepare the show.
On August 10, 2001, R&R changed its charts to Mediabase only data as well as its recurrent rule. With the chart change, the chart was now available a few days earlier, and thus AT40 decided to go back to the chart. The 1st AT40 show that used AT40's new chart was August 18, 2001.
Here is a summary of some of the differences between R&R and AT40 during the separation:
Seven songs were on AT40 but never made R&R's Top 40:
63% of all songs that debuted during the separation debuted first on AT40, then on R&R at least 1 week later.
The longest wait between a song debuting on AT40 and then R&R was 5 weeks, by American Hi-Fi's "Flavor of the Weak".
58% of all songs that fell off AT40 during the separation fell off earlier on AT40 than on R&R.
In terms of #1 songs: "Again" by Lenny Kravitz reached #1 on R&R but not on AT40, and "Survivor" by Destiny's Child reached #1 on AT40 but not R&R.
Many thanks to GordonUM from the former R&R Messagebords for compiling the above chart information.