Inter-laboratory comparison is one of the quality control procedures recommended by the Laboratory Accreditation Scheme of Malaysia (SAMM) MS ISO/IEC 17025:2005 (Clause 5.9.1) to monitor the validity of tests offered as services to clients. It provides one of the most expedient means of assessing the bias of one laboratory’s testing relative to other laboratories in the industry (Kishore Nadkarni et al., 2004). In fact, a combination of the data derived from statistical quality control and from the proficiency testing programs can satisfy the relatively new requirements for determining the measurement uncertainties.