I am trying to undertake some Z-Tests to see the level of effect in Standard Deviations of 8 separate treatments compared to a control mean.
However there is one thing that has constantly confused me in all my attempts. How do I set the mean rating of my control as the mean to compare all the other too?
Control T1 T2 T3 T4 T5 T6 T7 T8 Mean Ratings 4.74 -1.77 -1.88 7.77 4.08 6.90 13.66 17.13 19.44
The sample population for the control and each treatment is circa 105, is normally or approximately normally distributed and each participant contributed one score to each mean rating.
Calculating Z-Scores - Method 1
Using SPSS if I run a simple two tailed Z Test on the above using
Analyse > Descriptive Statistics > Descriptives I get:
N Minimum Maximum Mean Std. Deviation VAR00023 9 -1.88 19.44 7.7856 7.63361 Valid N (listwise) 9 Control: -.39897 T1: -1.25177 T2: -1.26618 T3: -.00204 T4: -.48543 T5: -.11601 T6: .76955 T7: 1.22412 T8: 1.52673
The above tells me that none of my treatments were significant at α=0.05
However this does not seem right as its including my control mean (4.74) in the equation.
Calculating Z-Scores - Method 2
If I run the same test without the control mean I get:
Descriptive Statistics N Minimum Maximum Mean Std. Deviation VAR00002 8 -1.88 19.44 8.1663 8.06883 Valid N (listwise) 8 T1: -1.23144 T2: -1.24507 T3: -.04911 T4: -.50642 T5: -.15693 T6: .68086 T7: 1.11091 T8: 1.39720
The above tells me much the same as the previous method, that none of my treatments were significant at α=0.05, however there are variations in each score
This does not seem right either as I have not specified the control mean (4.74) that I want all of the Treatments to be compared too?
Can anyone tell me which is the right track, or the closest to it, and where I might be going wrong?
Neither method I have tried appears to be right for the aforementioned reasons.
Thanks all for taking the time to look at this. I have struggled with this for a good few days now and would really appreciate if anyone could explain the correct way to undertake a Z-Test using the above data so that I can see where my reasoning has been going wrong. Thanks
NOTE: I understand from the answer to previous question I asked that I will have to correct for Family Wise Error Rate, that will be my next challenge to figure out, I just want to see if I can get this part figured out first.