BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem
TTP OnDemand GMAT Video Course
Featured Course

Target Test Prep GMAT OnDemand

Scott Woodbury-Stewart’s private virtual classroom — 400 hours of master-class video lessons for the GMAT Focus Edition.

  • 715+ score guarantee — highest in the industry (99th percentile)
  • 52 chapters · 1,500+ lessons · 4,000+ practice questions
  • 400 hours of video · 1,500+ instructor-led HD smartboard lessons
  • 300,000+ students accepted to Harvard, Stanford, Wharton, Booth & Sloan & more
  • 24/7 live support + weekly Zoom office hours with GMAT instructors
  • TTP AI Assist — 24/7 AI-powered virtual tutor for instant help
  • 1,200+ flashcards + AI-powered study assistant & daily calendar
  • OnDemand, LiveTeach & GMAT Bootcamp formats available
  • Also: GRE, SAT Math & Executive Assessment courses
  • MBA Admissions Consulting now available
  • 🏆 2025 EdTech Breakthrough Award: Test Prep Solution Provider of the Year
  • 200,000+ students served
  • 5-day free trial — $0 to start, no auto-billing, cancel anytime
★★★★★ 5.0 (559 reviews)
130-pt guarantee
$0 to start then $127/mo
Start Free 5-Day Trial Read 559 Reviews →

Mean and Standard Deviation

Expert replies
Post new topic Post Reply
Previous Topic Next Topic
User avatar
Master | Next Rank: 500 Posts
Posts: 351
Joined: Mon Jul 04, 2011 10:25 pmFollowed by: 4 members
Thanked: 57 times
by akhilsuhag » Sun Sep 04, 2011 1:21 pm
The average (arithmetic mean) of a normal distribution of a school's test scores is 65, and standard deviation of the distribution is 6.5. A student scoring a 78 on the exam is in what percentile of the school?

[spoiler]OA: 98[/spoiler]
Please press "thanks" if you think my post has helped you.. Cheers!!
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Source: — Problem Solving |
Legendary Member
Posts: 1084
Joined: Fri Apr 15, 2011 2:33 pmFollowed by: 21 members
Thanked: 158 times

by pemdas » Sun Sep 04, 2011 1:47 pm
The percentages are valid for any question involving a normal distribution, and one should memorize them as 34: 14 : 2. The percentages correspond to the 1st, 2nd, and 3rd standard deviations on each side of the mean. A student scoring 78 on the exam is placed two standard deviations on the right side of the mean, that is 50% (mean) + 34% + 14% = 98%
Success doesn't come overnight!
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Senior | Next Rank: 100 Posts
Posts: 47
Joined: Thu Jul 14, 2011 9:41 am
Thanked: 9 times
GMAT Score:650

by prashant.mishra » Tue Sep 06, 2011 7:08 am
Hi Pemdas,

Can you give a detailed explanation to the above logic ?
pemdas wrote:The percentages are valid for any question involving a normal distribution, and one should memorize them as 34: 14 : 2. The percentages correspond to the 1st, 2nd, and 3rd standard deviations on each side of the mean. A student scoring 78 on the exam is placed two standard deviations on the right side of the mean, that is 50% (mean) + 34% + 14% = 98%
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Newbie | Next Rank: 10 Posts
Posts: 2
Joined: Thu Sep 18, 2008 10:46 am

by karthi_crazy_mit » Tue Sep 06, 2011 8:20 am
can you please explain the solution.
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Junior | Next Rank: 30 Posts
Posts: 12
Joined: Tue Aug 30, 2011 7:55 pm
Thanked: 1 times

by jbivins » Tue Sep 06, 2011 11:25 am
Would this even be a GMAT question Since it requires knowledge of the normal distribution and standard deviation's relation in normal distributions?

https://www.regentsprep.org/Regents/math ... Lesson.htm
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Legendary Member
Posts: 1084
Joined: Fri Apr 15, 2011 2:33 pmFollowed by: 21 members
Thanked: 158 times

by pemdas » Tue Sep 06, 2011 12:43 pm
Standard deviation is not such a big deal as much as mean and median are concerned in GMAT exam. They are all central tendency measures studied at the college level and sometimes included in SAT advanced exam question sets. Why would normal distribution be mentioned here is to validate that the central tendency measures are relevant for their application. There is only one special relation of standard deviation to normal probability related to the pure statistical science and it was not mentioned here, z-score and the table of values for each degree variance.

As such, this is a valid GMAT question and will be tested at the medium to low-average difficulty levels of applied algebra section. There's nothing to be rounded as logic here, but one rule of 34-14-2 to memorize and it was described in my previous post.
jbivins wrote:Would this even be a GMAT question Since it requires knowledge of the normal distribution and standard deviation's relation in normal distributions?

https://www.regentsprep.org/Regents/math ... Lesson.htm
Success doesn't come overnight!
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 2623
Joined: Mon Jun 02, 2008 3:17 amFollowed by: 355 members
Location: Montreal
Thanked: 1090 times
GMAT Score:780

by Ian Stewart » Tue Sep 06, 2011 1:02 pm
pemdas wrote:

As such, this is a valid GMAT question and will be tested at the medium to low-average difficulty levels of applied algebra section.
No, this is definitely not a 'valid' GMAT question. Normal distributions are never tested on the GMAT. I've seen quite a few prep company questions about normal distributions and the 68-95-99 rule, and they are all completely irrelevant questions for GMAT test takers to study.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

ianstewartgmat.com
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Legendary Member
Posts: 1084
Joined: Fri Apr 15, 2011 2:33 pmFollowed by: 21 members
Thanked: 158 times

by pemdas » Tue Sep 06, 2011 1:18 pm
Ian, please check Princeton Review's manual for GMAT exam. It's exclusively for their students, version 8.0 and page # 96. Do you think guys at @PR waste their time and people's moneys teaching the students 34-14-2 stuff?


Ian Stewart wrote:
pemdas wrote:

As such, this is a valid GMAT question and will be tested at the medium to low-average difficulty levels of applied algebra section.
No, this is definitely not a 'valid' GMAT question. Normal distributions are never tested on the GMAT. I've seen quite a few prep company questions about normal distributions and the 68-95-99 rule, and they are all completely irrelevant questions for GMAT test takers to study.
Success doesn't come overnight!
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 2623
Joined: Mon Jun 02, 2008 3:17 amFollowed by: 355 members
Location: Montreal
Thanked: 1090 times
GMAT Score:780

by Ian Stewart » Tue Sep 06, 2011 3:12 pm
pemdas wrote:Ian, please check Princeton Review's manual for GMAT exam. It's exclusively for their students, version 8.0 and page # 96. Do you think guys at @PR waste their time and people's moneys teaching the students 34-14-2 stuff?
Yes, that is exactly what I think. And, as I've posted many times, several of their questions about those rules are mathematically nonsensical.

If you disagree with me, feel free to point me to even one official question which would require you to know the 68-95-99 "rule".
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

ianstewartgmat.com
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 16207
Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members
Location: Vancouver, BC
Thanked: 5254 times
GMAT Score:770

by Brent@GMATPrepNow » Tue Sep 06, 2011 5:38 pm
I'd like to echo Ian's point. Normal Distributions are not tested on the GMAT.

Questions like this cause students to either panic (if they don't know anything about Normal Distributions) and/or needlessly study a concept that they will never be tested on.

Cheers,
Brent

PS: Normal Distributions have recently been added to the GRE curriculum. Perhaps someone has confused GRE questions with GMAT questions(?)
Brent Hanneson - Creator of GMATPrepNow.com
Image
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Post new topic Post Reply
• Page 1 of 1