4. Model Formulation

4.1. Notation

\(N_{j k l}:\) the requirement of staff of category \(j\) in shift \(k\) in day \(l\),

\(n_{j}\) : the number of employees of category \(j\),

\(S_{w}: \quad\) the set of days of week \(w\),

\(S_{\text {off }_{(i, j)}}\) : the preferable set of days off of employee \(i\) of category \(j\),

\(\mathrm{AL}_{i j}\) : the set of annual leave days of employee \(i\) of category \(j\),

\(s_{1 w}\) : the first 5 days of each week \(w\),

\(s_{2 w}: \quad\) the 2 last days of each week (Thursday and Friday).