I have two groups, 23 Patients and 27 controls with a list for their age and the head circumference measurements. I want to do scatter plot for the Z score of age by the Z score of head circumference.
I will simplify the matter by the following table for 4 controls and 5 patients:
GROUP age head circumference
Patient 5 20 cm
Patient 7 19 cm
Patient 4 22 cm
Patient 8 17 cm
Patient 6 18 cm
Control 9 17 cm
Control 3 23 cm
Control 4 21 cm
Control 6 24 cm
My question is about the correct method in this case to calculate the Z score for age and head circumference between the groups (I will mention the possible ways that I am thinking about regarding how to calculate Z score for every age point for example)
1. Z score patients = { age - mean age (patients)}/STD(patients) and Z score controls = { age - mean age (controls)}/STD(controls)
2. Z score patients = { age - mean age (controls)}/STD(controls) and Z score controls = { age - mean age (patients)}/STD(patients)
3. Z score patients = { age - mean age (controls)}/STD(controls) and Z score controls = { age - mean age (controls)}/STD(controls)
The purpose of my question is to inquire that if I have the means for two groups and I want to compare the z score for the means between the groups using scatter plot for the z scores. In order to calculate the Z score for the first group is it correct to subtract the age (in my example) from the age mean of the second group then divide by the standard deviation of the age in the second group and vice versa?
Best Answer
Actually there is no difference between the three methods that you mentioned before and the results. Simply try to do scatter plot for the Z score by doing calculations with any of the previous methods and you will have the same results.