Stats

Question 1

Standard deviation estimations

Answer 1

68% - 1 SD
95% - 2 SD
99.7% - 3SD

Question 2

Inverse normal

Answer 2

If x<a given, then no change necessary
If x>a given, then turn into 1-a.
If 16<x<a given find area of x<16 then add to 16<x<a then do x<a with new area.

Question 3

What is z

Answer 3

Z is the number of SD above the mean
z= x-mean/standard deviation

Question 4

What to do if standard deviation or mean is unknown?

Answer 4

First write down probability in terms of X - P(X<20) = 0.8
Then write down probability in terms of Z - P(Z<a) = 0.8
Then use inverse normal to find value for a using 1 for the SD and 0 for the mean. a=0.8416
Then use equation that links X, Z, SD and mean - Z=X-mean/SD 0.8416= 20-mean/SD (here sub in either SD or mean given in equation-either one could be given)