Yukiho and Haruka are the most average idols and I can prove it.

[PluggedBus]

Age in years and birthdate:
1. Azusa: 21 (July 19th)
2. Ritsuko: 19 (June 23rd)
3. Takane: 18 (January 21st)
4. Haruka: 17 (April 3rd)
5. Makoto: 17 (August 29th)
6. Yukiho: 17 (December 24th)
7. Chihaya: 16 (February 25th)
8. Hibiki: 16 (October 10th)
9. Iori: 15 (May 5th)
10. Miki: 15 (November 23rd)
11. Yayoi: 14 (March 25th)
12. Ami: 13 (May 22nd)
13. Mami: 13 (May 22nd)
Average age: 16

Height in centimeters:
1. Takane: 169
2. Azusa: 168
3. Chihaya: 162
4. Miki: 161
5. Makoto: 159
6. Haruka: 158
7. Ami: 158
8. Mami: 158
9. Ritsuko: 156
10. Yukiho: 155
11. Iori: 153
12. Hibiki: 152
13. Yayoi: 145
Average height: 158

Weight in kilograms:
1. Takane: 49
2. Azusa: 48
3. Haruka: 46
4. Miki: 45
5. Makoto: 44
6. Ritsuko: 43
7. Ami: 42
8. Mami: 42
9. Yukiho: 42
10. Chihaya: 41
11. Hibiki: 41
12. Iori: 40
13. Yayoi: 37
Average weight: 43

Bust in centimeters:
1. Azusa: 91
2. Takane: 90
3. Miki: 86
4. Ritsuko: 85
5. Haruka: 83
6. Hibiki: 83
7. Yukiho: 81
8. Ami: 78
9. Mami: 78
10. Iori: 77
11. Makoto: 75
12. Yayoi: 74
13. Chihaya: 72
Average bust: 81

Waist in centimeters:
1. Takane: 62
2. Azusa: 59
3. Makoto: 57
4. Ritsuko: 57
5. Haruka: 56
6. Hibiki: 56
7. Yukiho: 56
8. Chihaya: 55
9. Ami: 55
10. Mami: 55
11. Miki: 55
12. Iori: 54
13. Yayoi: 54
Average waist: 56

Hips in centimeters:
1. Takane: 92
2. Azusa: 86
3. Ritsuko: 85
4. Miki: 83
5. Haruka: 82
6. Yukiho: 81
7. Hibiki: 80
8. Iori: 79
9. Chihaya: 78
10. Makoto: 78
11. Yayoi: 78
12. Ami: 77
13. Mami: 77
Average hips: 81

Most average girls:
1. Yukiho: 5 points away from completely average
2. Haruka: 6 points away from completely average
3. Hibiki: 10 points away from completely average
4. Ami: 12 points away from completely average
5. Mami: 12 points away from completely average
6. Makoto: 13 points away from completely average
7. Miki: 14 points away from completely average
8. Ritsuko: 14 points away from completely average
9. Iori: 17 points away from completely average
10. Chihaya: 19 points away from completely average
11. Yayoi: 33 points away from completely average
12. Azusa: 38 points away from completely average
13. Takane: 45 points away from completely average

Edit: Oh, I forgot to mention that the reason I included the birthdates is because fans like to celebrate those. Think of it as a little bonus.

Edit 2: Cael K. indirectly pointed out that I hadn't said if I looked at their stats and thought that something was up or if I whimsically decided to type up all the numbers to find something. It was the latter.

[Scotty]

I agree with all of this.

[SalemJeanette]

Baka Ribbon-chan is still pretty cool.

[Haldo]

Harkua  being really average hardly surprises me

[kozu]

Guys, this just means:

Azusa da best

[Cael K.]

Everything's weighted at x1.0? No considering standard deviation, even?

Azusa da best

Yes.

[PluggedBus]

Everything's weighted at x1.0? No considering standard deviation, even?

I have no idea what any of that means. This is elementary school level math. I didn't attend high school.

[Cael K.]

Oh, whoops. Sorry dude.

Doesn't make it any less cool that you came across this. Pretty noticeable, you are.

[PluggedBus]

Oh, whoops. Sorry dude.

Doesn't make it any less cool that you came across this. Pretty noticeable, you are.

I didn't "come across it". I typed it up myself in notepad. I cross referenced their profile pages on the official iDOLM@STER website and used the calculator that is built into Windows to do the math. It took around maybe an hour or so altogether.

[Cael K.]

I probably should've said that it's a pretty cool discovery you made there. ^^;

Come across it, in the sense that you looked at the numbers and thought that something was up or that you wanted to find something out, then did the math. It's all cool. ^^;

[TweenDoriru]

and yet Yukiho creams the rest of the idols as fan favorite, and Haruka is the "main" iM@S girl. Weird..."average" must be pretty darn good.

[ikuto-sama]

and yet Yukiho creams the rest of the idols as fan favorite, and Haruka is the "main" iM@S girl. Weird..."average" must be pretty darn good.

Yukiho is just adorable. Everyone loves crybaby girls. When they're upset, you're like "Aww" and when they finally take a stand for themselves and are assertive, you cheer for them. They're fun characters to watch develop. Although I still miss Yukiho's old voice...but I try to look past that now. XD 
Based on personality average-ness, Haruka would probably end up being the winner.

[Sev]

Interesting math pretty nice of you to do all that for us. Though to classify them as totally "average" you'd have to compare them with the national averages for Japanese women. In this particular case they'd only be considered "the average" out of the total group of 13 girls. The challenge with comparing them nationally would be finding the statistics since I'm pretty sure most Japanese women don't just submit their three sizes to some convenient national census.

[ProjectAria]

Guys, this just means:

Azusa da best

But with all of this is shows Takane is the best not Azusa.

[Saturos]

But with all of this is shows Takane is the best not Azusa.

Guys, this just means:

Azusa da best

HEAVEN OR HELL.
FIGHT!