calvin1984 wrote:
Sorry to bring up an old topic, just wanted to clarify some things.
For question 2, can I assume that a constant added value will never change the value of the SD?
Yes, if we add (or subtract) a constant to each term in a set:
Mean will increase (or decrease) by the same constant.
SD will not change.For example: if the standard deviation of set A={1, 11, 15} is
d then the standard deviation of the following sets B={6, 16, 20} and C={0, 10, 14} is also
d (set B obtained by adding 5 to each of the element of set A and set C is obtained by subtracting -1 to each of the element of set A).
On the other hand if we increase or decrease each term in a set by the same percent (multiply by some constant):
Mean will increase or decrease by the same percent.
SD will increase or decrease by the same percent.For example: if the standard deviation of set A={1, 11, 15} is
d then the standard deviation of the following sets B={2, 32, 30} is
2d (set B obtained by multiplying each of the element of set A by 2 (increasing by 100%)).
More tips on SD:
ps-questions-about-standard-deviation-85897.html (collection of PS questions on SD)
lately-many-questions-were-asked-about-the-standard-85896.html (collection of DS questions on SD)