This document is a technical report detailing the Thomas Aptitude assessment, also referred to as the General Intelligence Assessment (GIA). It outlines the assessment's purpose, which is to measure cognitive abilities and trainability across five core areas: fluid intelligence, concentration, response to training, mental processing speed, and speed of development. The assessment consists of five sub-tasks, each targeting specific cognitive skills, including reasoning, perceptual speed, numerical accuracy, word meaning, and spatial visualization. The report further discusses the theoretical foundations of the assessment, referencing various psychological theories and cognitive task paradigms. It explains the concept of trainability and its significance in predicting work outcomes, supported by meta-analysis findings that link cognitive ability to job performance. The document also addresses the validity of the Aptitude assessment through concurrent and predictive validity studies, demonstrating its effectiveness in various professional contexts.