Describe a fictional study that requires a mixed model design with one categorical and one continuous predictor, including the units in which each variable was measured (e.g., cm, ml, m, l, etc). Describe the rationale for the need of a mixed model analysis in the context of your problem. The data have to be generated using R. Code for generating the data also needs to be included in the code file; but because the data were generated in R, you don't need to save them in a csv file and reload it. Data should be generated in such a way that it fits best the random intercept and slope mixed model. Report all the necessary statistical assessments to make sure that your data fit the model requirements and that your model fits best with the random intercept and slope model than with a random intercept only model. Report results and provide a small interpretation in line with your fictional example.