Validation refers to the formal assessment, or rigorous set of policies that challenge the specific objectives of a test method or model with regard to its relevance and reliability. This in turn provides the foundation

to facilitate regulatory adoption and Autophagy signaling pathway inhibitor acceptance (Corvi et al., 2006; Stephens and Mak, 2013). Relevance refers to the extent to which a test or model correctly predicts/measures the biological effect of interest; reliability is the degree to which the data in the protocol is reproducible within the guidelines or protocol of the method (Barile, 2010). Most protocols undergo a pre-validation stage, designed to prepare a test model or assay for further progression into a formal validation study. These may involve intra-laboratory studies to address protocol optimization (Phase I), transferability (Phase II) and performance (Phase III) (Van Goethem et al., p38 MAPK pathway 2006), so that prior agreements can be made on detailed protocols that prepares and aids the test model or test in the formal validation process.

There are typically two types of validation study: prospective and retrospective (Kandárová and Letašiová, 2011) and a combination of these approaches are usually applied in the formal validation process (Hartung et al., 2004). Prospective studies involve the generation of new data, whilst retrospective studies re-assess existing data under standardized, controlled conditions. ECVAM have proposed a modular validation assessment (Hartung et al., 2004), comprised of 7 modules aimed at determining the performance characteristics, advantages and limitations of a model or test for a specific purpose (Kandárová and Letašiová, 2011). The modules are: (i) test definition, where the scientific objective of the model or test, a mechanistic basis, a specific protocol

including all standard operating procedures with clearly defined endpoints, see more methods of results interpretation via prediction models and specific controls used must be clearly defined; (ii) intra-laboratory variability assessment, to determine potential variations in data incurred due to different operators carrying out the protocol within the same laboratory set-up. This assessment stage is usually not so problematic, since laboratories developing a model or test would usually abandon or modify an irreproducible protocol prior to assessment submission ( Ubels and Clousing, 2005); (iii) transferability, to demonstrate that the test can be repeated in different laboratory set-ups. In the case of in silico models, this is the ability of different operators to reproduce the model definition and predictions, which is often dependent upon the strength of the explanatory documentation provided; (iv) inter-laboratory variability, whereby three to four laboratories are typically asked to test a defined number of substances using the assessed method or model to highlight discrepancies.