Persons holding a PhD and working in public institutions (university, research centre etc.) have the right to apply for an extra taxfree income. The idea is to attract and to keep researchers in Mexico. The S.N.I. is maintained by CONACyT, e.g. the Mexican equivalent to the American NSF or German DFG.
The S.N.I. knows four levels, C (candidate), I, II, and III. The monthly income is about 500, 750, 1200, 1700 USD (III is estimated; 19 Mexican Pesos for one USD). These values are increased yearly by about 4% to cope with the inflation.
The S.N.I. is granted for three years, but the time span may increase if a person remains in the same level. The evaluation is not transparent and personal relationship may help to receive or maintain a certain level even if there does not exist an equivalent scientific output. Main focus is on papers listed in the Science Citation Index, patents, books, and contributions in meetings. Increasingly also other activities as the formation of working groups, students, classes, etc. are included for evaluation.