With the February 8, 2003 show, the AC version of American Top 20 changed charts. Instead of using Radio & Records' AC chart, AT20 AC now uses an unpublished Mediabase AC chart. The new chart is different than the R&R chart in the following ways: 1) Songs played on the syndicated daily "Delilah" show are not counted toward the chart, and 2) The chart is ranked based on audience impressions rather than just how many times a song is played. No information is available for the recurrent rules of the new chart. This information came from a staff member of AT20 and is listed here with permission.
With the November 2nd 2001 issue, Radio & Records changed its recurrent rule again. For the CHR/Pop chart, which AT40 currently follows, the recurrent rule reverts back to 20 weeks on and below #20. This is the same rule that R&R used before its charts changed back in August.
For the AT20 shows, the recurrent rule changed to 20 weeks on and below #15. This is a new rule for these charts.
For a history of the chart saga and some differences between R&R & AT40 during the separation, see below.
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 is 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 R&R Messagebords for compiling the above chart information.