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

Redeem

Target Test Prep · GMAT

Choose how you want to prepare

Learn live with an expert or move at your own pace. Every option includes the complete TTP study system.

★★★★★5.0559 reviews
GMATLiveTeach 7 seats left
Chris Peckover
NEXT LIVE COHORT

Oct 13 to Jan 7, 2027

with Chris Peckover

Schedule
Tue, Thu · 8:00 to 10:00 PM ET
Included
40 live hours + 6 months of GMAT OnDemand
  • Live instruction and real-time questions
  • Class recordings and assigned practice
View class & enroll
Limited cohort · enrollment openTarget Test Prep
EALiveTeach 5 seats left
Logan Thompson
EXECUTIVE ASSESSMENT

Sep 6 to Dec 6, 2026

with Logan Thompson

Schedule
Sun · 9:30 AM to 12:30 PM ET
Included
Live EA class + 6 months of EA OnDemand
  • Expert-led weekly online sessions
  • EA Masterclass access between classes
View EA class & enroll
Limited cohort · enrollment openTarget Test Prep
GMATOnDemand Start anytime
SELF-PACED MASTERCLASS

Target Test Prep GMAT OnDemand

Complete access from day one. Study on your schedule.

130-point score guarantee
$0to start then $127/mo
  • Personalized study plan and analytics
  • Thousands of lessons and practice questions
Start free 5-day trial
Read 559 reviewsTarget Test Prep

Compare the format, schedule, and included access before enrolling. Prices and seat counts shown reflect the supplied offer details.

Can we add standard deviations ?

Expert replies
Post new topic Post Reply
Previous Topic Next Topic
Master | Next Rank: 500 Posts
Posts: 102
Joined: Sat Mar 15, 2008 4:03 am
Thanked: 4 times
by wilderness » Tue Jul 15, 2008 1:56 pm
Hi,

Set A and B are 2 sets of numbers.

A has a standard deviation 3 and a mean 5.

B has the same mean but standard deviation 4.

Can we find the standard deviation of the set A U B ?

If no, then is it possible with the following additional information

1. Both sets have the same number of members each (lets say 4)
2. Both sets have distinct members (no number is common to both sets).

Thanks.
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Source: — Problem Solving |
User avatar
GMAT Instructor
Posts: 189
Joined: Thu May 01, 2008 10:55 pmFollowed by: 1 member
Location: Seattle, WA
Thanked: 25 times
GMAT Score:750+

by VP_Tatiana » Thu Jul 17, 2008 11:59 am
We cannot find the SD of A U B with the original information. We determine this by thinking about an extreme counter example. If A has a million members, and B has one member, the SD of the union will be nearly equal to the SD of A. However, if B is the much larger set, then the SD of the union will be much closer to B.

By telling us that the sets contain equal members, we know we can weight the variances equally when calculating the combined SD. sqrt(variance A) = 3, and sqrt(variance B) = 4. The SD of A U B would just be sqrt ((Va + Vb)/2).

Statement 2 does not give us any useful information. Duplicate entries become irrelevant as we do the variance calculation, and don't affect the calculation in a different way from unique entries.

So, statement 1 alone is sufficient information.

Best wishes,

Tatiana
Tatiana Becker | GMAT Instructor | Veritas Prep
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Master | Next Rank: 500 Posts
Posts: 102
Joined: Sat Mar 15, 2008 4:03 am
Thanked: 4 times

by wilderness » Thu Jul 17, 2008 12:08 pm
Thanks VP_Tatiana
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Post new topic Post Reply
• Page 1 of 1