## 1 Introduction  
A reference material (RM) is a requirement for different kinds of measurements because it allows different methods to be established for quantitation adjustment, reproducibility evaluation, and assessment of adequacy of the method for the analyte measurements [1; 2]. Today, there is still a need to develop RMs in many applications. The procedure and definitions are clear since there are some regulations [3; 4]. However, there are many difficulties in developing RMs in some cases due to variability of the matrix, its stability and interaction with compounds, and difficulty in synthesizing compounds with adequate purity, etc. A RM is defined as a material, sufficiently homogeneous and stable with respect to one or more specified properties, which has been established to be fit for its intended use in a measurement process [5]. There are two categories, RMs and certified reference materials (CRMs), which differ in the certificate that establishes the traceability to accurate realization and for which the certificate values are accompanied by an uncertainty at a defined level of confidence [3]. Thus, the process for the certification of a RM/CRM is well-established by a process that ensures traceability to accurate realization of the unit in which the property values are neatly expressed. RMs can be produced by institutions that are competent according to ISO 34 [4]. There are methods where the implementation of RMs are underdeveloped; for instance, methods on sensory assessment rarely use RMs in a strict sense according to the ISO definition [3]. However, sensory assessment, like any other metrology, requires that RMs provide qualitative and quantitative results. Furthermore, methods on sensory assessment of foods are not always included in trade standards and regulations or they are not described in detail. In the case of virgin olive oil (VOO), sensory assessment is a standard method, and its application is mandatory in business activities according to International Olive Council (IOC) norms and other national and international regulations [6; 7]. In fact, there is a profuse legislation about the authenticity and quality characteristics of VOO categories [6] that is accompanied by series of analytical methodologies to determine the values of the parameters characterizing this edible oil. The laboratories that implement the analytical methods must demonstrate their proficiency in applying testing methods, following the procedures described by EU [8] and IOC [9], with trials that ensure the validity of the results. That validation process should be carried out with RMs, ideally CRMs, but there are not any CRMs to be used in the evaluation of sensory testing laboratories or for training tasters. Even if there is a good agreement among international regulations regarding the organoleptic quality of VOO [6; 10] and the protocol for training tasters and sensory tasting laboratories [11; 12; 13], there are no RMs for sensory defects that are commonly detected in non-extra-VOOs (oils from virgin olive oil and lampante olive oil categories) to be used in the sensory analytical test for panel training and control. Instead, some natural VOOs are regularly selected as being representative of each sensory defect. They are considered as reference oils although their availability is limited, they are not homogeneous year after year, and other secondary defects can be present as well. In 1991, the IOC method was included in European regulations and obtained legal validity to establish the quality grade of the product that included only three categories of VOOs: extra-virgin (EV), virgin (V), and lampante (L). Thus, in order to classify VOOs, a sensory profile sheet reports different oriented segments for assessment of intensity of the sensory descriptors in VOOs. The values, expressed as centimeters, are statistically processed to calculate the median of each positive (fruity, bitter and pungent) and negative attribute (main defects: rusty-muddy, winey-vinegary, rancid, musty, frostbiten olives) [11; 12].