Information System Quality (ISQ) management discipline requires a set of assessment mechanisms to evaluate external quality characteristics that are influenced by the environmental parameters and impacted by the ecosystem factors. The present paper suggests a new assessment oriented model that takes into consideration all facets of each external quality feature. The proposed model, named RatQual, gives a hierarchical categorization for quality. RatQual is designed to quantify dependent-environment qualities by considering internal, external and in use aspects. This model is supported by a tool that automates the assessment process. This tool gives assistance in quality evolution planning and serves for periodical monitoring operations used to enhance and improve information system quality.