## Multilevel models :

Multilevel models, also known as hierarchical models or mixed-effects models, are a type of regression analysis that allows for both fixed and random effects. This means that the model can account for both variability that is inherent to the data (random effects) as well as variability that is due to external factors (fixed effects).

One example of a multilevel model is a study that aims to understand the relationship between students’ math test scores and the type of teaching method used in their classroom. In this case, the students’ test scores would be the dependent variable, and the teaching method would be the independent variable. However, the study also includes a third variable, the school that the students attend. This variable is included as a random effect because it is likely that schools will differ in their overall levels of education, which could affect the students’ test scores. By including this variable as a random effect, the multilevel model can account for this inherent variability in the data.

Another example of a multilevel model is a study that examines the relationship between the amount of physical activity a person engages in and their overall health. In this case, the person’s level of physical activity would be the independent variable, and their health would be the dependent variable. However, the study also includes a third variable, the person’s age. This variable is included as a random effect because it is likely that people of different ages will have different baseline levels of health, which could affect the relationship between physical activity and health. By including this variable as a random effect, the multilevel model can account for this inherent variability in the data.

Overall, multilevel models are a powerful tool for regression analysis because they allow researchers to account for both fixed and random effects in their data. This can help to provide a more accurate and comprehensive understanding of the relationship between different variables.