I have 99 patients divided into subgroups according to the following variables:
Their syntax score (which measures how severe the obstruction in coronary arteries is). Values of this variable are between 10.2 and 41 and are classified into three levels, first 1-22, second 23 - 32, third 33 or above. An increase in the syntax score means worse heart condition.
MPI, which is myocardial performance index. Values of this variable are between 0.25 and 1.2. Increase in MPI also means worse heart condition.
Theoretically there is association between syntax score and MPI, so that increased syntax score should cause increased MPI, but when I did statistical analysis using SPSS there was no significant association.
Note that 35 patients from these 99 have normal coronary arteries so they are excluded from statistical analysis.
I did analysis using correlation, chi-square and ANOVA where I dealt with values in different ways. In correlation, I dealt with both syntax score and MPI as scale (continuous) variables, while in chi-squared, I dealt with both syntax score and MPI as nominal variables. In ANOVA, I dealt with syntax score as nominal variable and MPI as scale variable.
What is the most suitable statistical analysis?