I am doing a research and want to analyze factors that are associated with dental erosion using binary regression analysis. Dental erosion (dependent variable) was graded using a scale criteria with 5 points (0= No erosion, 1=mild erosion, 2=moderate erosion, 3=severe erosion, 4=very severe erosion). No one in the sample had grade zero. Therefore, because severe and very severe erosion are the most important clinically I am interested to see which factors are associated with occurrence of severe and very severe erosion (grades 3 and 4). The data were collected for 3 age groups (5, 13 and 18 years old children). I have the following question:
- I dichotomized the dependent variable into two groups: group with grades (0,1) and other group with grades (3,4) and excluded grade 2 to create good contrasting groups. This resulted in massive drop in sample size but resulted in high odds ratios. Is that correct to do or should I include grade 2 with the first group as (0,1,2)?