Nowadays, with developing technology, it is possible to collect and record a large number of variables for the underlying experimental units in different researches. If these variables are collected over time for different individuals, the resulting data is called longitudinal data. Longitudinal studies are often used in Econometric, Medicine and social sciences. One important source of dependency in these types of data sets is the intra-class correlation which is created due to the effects of individuals’ characteristics on their corresponding observations. Mixed-effects models are regression models which are considered to take into account the intra-class correlation in longitudinal data through considering random effects in the structure of the underlying models.
