Question: What was the average number of viewers per episode of Grey's Anatomy during the first four seasons?
About the article "Grey's Anatomy", section "U.S. television ratings".

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On , the backdoor pilot of "Private Practice" aired on ABC, the pilot was in the double episode of "Grey's Anatomy" entitled "The Other Side of This Life", and gave a brief description of the characters in "Private Practice".
The cast included Amy Brenneman, Paul Adelstein, Tim Daly, Taye Diggs, Chris Lowell, and Merrin Dungey.
The 2-hour episode averaged 21 million viewers, 1.9 million more viewers than "Grey's Anatomy's" third season 19.1 million viewers-per-episode average at the time of its airing.
The episode was also No.
1 in the 9:00 pm and 10:00 pm timeslots.
On , it was announced, on "The Ellen DeGeneres Show", that "Private Practice" was part of ABC's 2007 fall line-up.
The first TV promo for the series aired during the season finale of "Grey's Anatomy", on .
The series premiere aired on , averaging 14.41 million viewers being the most watched show in its time slot.
The series premiered on British television on , on LIVING.
It was later moved to Thursday at 10:00 pm after "Grey's Anatomy" during mid-season 2009, to make room for the return of "Lost". 
"Greys Anatomy" has received high viewership and ratings since its debut.
The first four seasons of the program each ranked in the top ten among all viewers, reaching its peak Nielsen ratings in the second season, attracting an average of 19.44 million viewers per episode, and ranking at fifth place overall.
Following the show's time-slot being relocated, overall rankings steadily declined, dropping below the top ten in its fifth season.
"Grey's Anatomy" made its greatest fall from its sixth to seventh season, slipping from seventeenth place to thirty-first.
The series is on a steady decline in terms of overall viewership and rankings, yet "Grey's Anatomy" still holds value in charts when numbers are pulled from the digital video recorder (DVR).
It was the most recorded show between 2007 and 2011, based on cumulative totals, and has been for several years in a row.
The most-watched episode of the series is "It's the End of the World", with 37.88 million viewers, aided by a lead-in from Super Bowl XL.
In contrast, the least-watched episode is "The Lion Sleeps Tonight", which garnered 8.19 million viewers—a 30% decrease from "It's the End of the World".
In 2011, "Grey's Anatomy" was named the fourth-highest revenue earning show of the year, with US$2.67 million per half hour, behind "Desperate Housewives", "Two and a Half Men", and "American Idol" at the top.
In 2012, the show was the fifth-highest revenue earning show of the year, with US$2.75 million per half hour, behind "Glee", "Two and a Half Men", "The X Factor (U.S.)" and "American Idol".
While "Grey's Anatomy" is no longer ranked in the top numbers for overall ratings, the show's ranking in the key 18–49 demographic has remained high.
As of season eight, the series is the highest-rated drama on television in the target demographic.
Below is a table of "Grey's Anatomy"s seasonal rankings in the U.S. television market, based on average total viewers per episode.
Each U.S. network television season starts in September and ends in late May, which coincides with the completion of May sweeps.
The second season of "Grey's Anatomy" ended with an average of 19.440 million viewers per episode and a 6.9 rating share for in the 18–49 demographic, determining the series to finish in the fifth place out of all the 100 television shows in the season.
Due to its high ratings, the series received a full third season renewal for the fall prime time line up.
In response to numerous fan complaints regarding scheduling during the previous seasons, the American Broadcasting Company decided to do major changes in the season.
After two seasons of airing as a lead-out to fellow ABC series "Desperate Housewives", the network decided to move "Grey's Anatomy" to 9:00 ET in the Thursday night time slot, dominated by "", where the series began airing as a lead-in to "Ugly Betty", which aired in the time slot from its first season, until the conclusion of the third in 2009.
The show maintained its position as a top ten series and became the eighth most watched program in the season, with an average of 19.220 million viewers per episode.
The highest-rated episode of the season was the seventeenth, the highly anticipated conclusion of a three story arc, which was watched by 27.390, receiving a 9.7 rating, a #4 ranking in the week and a #1 ranking in the time slot.
The episode outperformed CSI's "", which ranked seventh with a 7.7 rating and 21.780 million viewers tuning in.
The lowest-rated episode of its ninth, which was watched by 18.510 million viewers, ranking third in the week with a 6.5 rating, outperforming "CSI" Thanksgiving special episode, "", watched by 17.170 million viewers with a 6.1 rating and #4 ranking.
The season premiere was watched by 25.410 million viewers and received 9.0 rating after being ranked #1 in both the time slot and the week.
The number of viewers increased significantly compared to the previous season premiere, which was watched by 18.980 million viewers and received a 6.8 rating.
"Time Has Come Today" also outperformed the previous season finale, which was watched by 22.500 million viewers and was rated 8.0.
The season finale was watched by 25.570 million viewers and received a 8.0 rating, ranking third in the week after "American Idol".
Wayne Firedman of "Media Daily News" described the move from the Sunday night time slot to Thursdays as "the network's boldest and biggest move".
He also expressed concerns regarding the tough competition the series will face, due to airing against CBS Network's "CSI".
Stephen McPherson of ABC Entertainment explained the reason for the change: "To have all hits on Sunday night doesn't help us.
We wanted to be aggressive". 
The first episode was watched by 1.877 million viewers, an 8.8% audience share, ahead of the first episode of "Facejacker" (1.186m) on Channel 4, and an episode of "Grey's Anatomy" (0.853m) on Five. 
In spite of numerous fan complaints regarding the previous season, the American Broadcasting Company decided to keep "Desperate Housewives" in its original Sunday night timeslot.
However, after two years of airing as a lead-in to fellow ABC series "Grey's Anatomy", the series would be followed by "Brothers & Sisters", then in its first season, but would remain the lead-out to '.
The show maintained its position as a top ten series and became the tenth most-watched program for the 2006-07 American television season, with an average of 17.5 million viewers per episode.
Ratings for the series fell considerably during the season, with a 22 percent loss of viewership toward the end of the season.
The highest-rated episode of the season was the season premiere, with 24.09 million viewers tuning in and 8.5 rating, ranking second in the week after the third season premiere of "Grey's Anatomy", which was watched by 25.41 million viewers and received a 9.0 rating.
The episode showed a decrease in ratings compared to the previous season premiere, which received a 10.1 rating and was watched by 28.36 million viewers, almost five more million viewers than "Listen to the Rain on the Roof".
Although the episode attracted less viewers than "Grey's Anatomy" "Time Has Come Today", the episode outperformed ', which would eventually become one of the most-watched series of the 2006–2007 season.
The lowest-rated episode was the nineteenth, watched by 15.91 million viewers, receiving a 5.6 rating.
It was the second most watched episode of an ABC series in the certain week, only after "Grey's Anatomy" "Time After Time", which was watched by 21.12 million viewers, and ranked fourth in the week with a 7.5 rating.
The season finale was watched by 18.82 million viewers and ranked seventh in the week, scoring a 6.6 rating.
There was a significant decrease in the number of viewers, compared to the previous season finale, which attracted almost seven more million viewers, receiving a 8.6 rating.