*Article* **Interlaboratory Comparison as a Source of Information for the Product Evaluation Process. Case Study of Ceramic Tiles Adhesives**

**Cristina Stancu <sup>1</sup> and Jacek Michalak 2,\***


**Abstract:** In this study, the results obtained by 19 laboratories participating in 2 editions of the interlaboratory comparison (ILC) determining 2 properties of ceramic tiles adhesives (CTAs), i.e., initial tensile adhesion strength and tensile adhesion strength after water immersion following EN 12004, were analyzed. The results show that participating laboratories maintain a constant quality of their work. The use of z-score analysis, under ISO 13528, allows for classifying 89.5% to 100% of laboratories as satisfactory, depending on the measurement's kind and edition. The remaining laboratories are classified as questionable. The investigation of the predominant mode of failure of the CTA's samples tested in the two editions shows significant differences. From the perspective of laboratories, the goal of the ILC has been achieved. From the standpoint of a manufacturer who evaluates a product's properties when placing it on the market, the results indicate the necessity of a particular treatment of the product evaluation process because the variability of the obtained results is significant. It increases the possibility of the product failing to meet the assessment criteria verified by the construction market supervision authorities. The manufacturer must consider all possible variations in the risk analysis, including the ILC results, to improve the assessment process of CTAs.

**Keywords:** interlaboratory comparison (ILC); proficiency testing (PT); risk analysis; measurements uncertainty (MU); ceramic tiles adhesive (CTA); assessment and verification of constancy of performance (AVCP); construction product; market surveillance
