$$ y = u_i + \varepsilon_{ij} \tag{1} $$
In the equation (1), \(y\) is the dependent variable, the change of the microtubule's length. \(y_{ij}\) is the \(j\) observed value of the independent variable under the \(i\) level. is the mean of dependent variable under the \(i\) level. stands for the residual between dependent variable’s value and it’s mean value, also obey the normal distribution \(N(0, \sigma_i ^2)\)