Unit and achievement standards represent the two kinds of standard used in NCEA. Both use criterion-based marking, which means students need to meet the specified criteria for each grade level to achieve at that level. However, unit standards are 'competency based' whereas achievement standards derive from the New Zealand Curriculum. Most unit standards use a simple Achieved/Not Achieved system, whereas achievement standards use a four-grade scale: Not Achieved (N), Achieved (A), Merit (M) and Excellence (E). Furthermore, each standard is assigned a particular credit value. For instance, Standard 91394 (Analyse ideas and values of the classical world) is worth four credits while Standard 91587 (Apply systems of simultaneous equations in solving problems) is worth 3 credits. All passing marks for a standard receive the same number of credits, such that one candidate might achieve 4 credits in AS 91394 when another achieves 4 credits in AS 91394 with Excellence. Due to criterion-based assessment, attainment in standards is independent.