Evidence based decision making is essential for enabling improved student learning. Teacher motivations and beliefs about the types and use of data are critical determinants of decision making. Our research explored the types of data teachers use and consider valuable when measuring improvement in student learning. Findings from 294 teachers from 44 schools in the Catholic (non-government) sector in New South Wales (NSW) Australia, found that data are used to inform the sequence of learning and to identify and address student needs. Both quantitative and qualitative data sources are applied to make claims of improvement. Factors such as ability, gender, school role, and school sector (primary or secondary) impact the selection and use of data. Our recommendations include the need to focus on professional learning to develop practising and preservice teachers' data literacy skills, ranging from statistical literacy to the identification and selection of appropriate data to substantiate claims of improvement.