Bacteria were measured on surfaces over time after using two different types of cleaning methods (A or B). They were measured using a quantitative method but the results have been reported as categorical:
- No growth (0 colonies),
- scanty growth (0< x<2.5)
- low growth(2.5 < x <12),
- medium growth (12 < x <40) and
- high growth(>40).
I've used the mid-point of each category as a continuous response variable in the following regression model: colonies~Cleaning* Time *Surface
But the response is only semi-quantitative and the steps between the categories are uneven. IS the model still valid using linear regression in R?