I am running my data using Polynomial Regression. The variables are all measured on 6-point Likert-type scales. I run Polynomial Regression in SPSS and and I center the predictors at 3.5 (which is the midpoint of 6-point Likert-type scales). The mean values of all variables are ranged from 3.8-4.2. However, my constant value is 5.78 and it seems to be too high as from my understanding constant value helps to predict the mean value of the outcome variable when the predictors are 0. The constant value is 4.2 when I didn't add the control variables.
My questions are:
1. From the graph above, the highest point to plot on Z-axis is 11.67. Since my variables are all measured on 6-point Likert-type scale, is this abnormal?
2. What does high constant value imply? I did double check my SPSS syntax and every step of the progress, yet still unable to get any insight
3. Is there any special way to deal with control variables in Polynomial Regression? The way I do it is just adding them with independent variables
Sorry if there are grammar errors as English is not my mother tongue. I urgently need answers for these. Please help