Standard deviation

[andrew.ng]

A list of 4 numbers a, b, c, d has standard deviation equal to that of which of the following list?

1) |a|,|b|,|c|,|d|
2) a+1, b+1, c+1, d+1
3) 5a, 5b, 5c, 5d
4) a^4,b^4,c^4,d^4
5) 2a+1, 2b+1, 2c+1, 2d+1

[mp2437]

Choice B. Mean will change with a uniform shift in all numbers, but standard deviation doesn't.

[khushboo143]

i came acroos one mor std dev question...couldn solve it..

A certain list of 100 data has an average (arithmethic mean) 6 and std dev d ,where d is positive. Which of the following data when added to the list must result in the list of 102 with std dev lass than d?
-6 &0
0&0
0&6
0&12
6&6

Correct ans is 6&6

[mehravikas]

There are some rules of thumb for standard deviation. You don't need to calculate SD on gmat.

1. if you add data that is farther from the mean, the standard deviation will increase
2. if you add data that is closer to the mean, the standard deviation will decrease

i came acroos one mor std dev question...couldn solve it..

A certain list of 100 data has an average (arithmethic mean) 6 and std dev d ,where d is positive. Which of the following data when added to the list must result in the list of 102 with std dev lass than d?
-6 &0
0&0
0&6
0&12
6&6

Correct ans is 6&6