All around us are examples of people and organizations with good information making bad decisions. Consider the case of Blockbuster. Less than a decade ago, Blockbuster had 8,000 stores in 17 countries and millions of customers. It was awash in data, including customer surveys that said people disliked going to video stores and hated late fees. Blockbuster failed to change with the times--and went bankrupt in 2010 (Satell, 2014). Accounts of what happened inside Blockbuster reveal striking parallels to observations of school data teams. These observations show that putting data in teachers' hands doesn't guarantee better student performance. Many obstacles impede the effective use of data in schools, including aspects of the data themselves and the mindsets of those expected to act on the data. This article contributes to the dialogue regarding school culture and the idea that even if teachers in a particular school receive timely data and are skilled enough to analyze it there are some school cultures that can seriously impede the ability to use data effectively. The good news however is that there are also schools where teacher teams dig deeply into data, question one another respectfully, and reflected upon their own practice. The article reports that what appears to make the difference is a principal who clearly defines the purpose for data analysis and creates a "we" attitude at the school. These strong leaders create and model norms for data conversations, specifying what materials -and attitudes-teachers need to bring to meetings. Highly effective principals set up ways teachers would hold one another accountable, argue productively in a safe and confidential environment, and ensure that conversations about students never turn to "nitpicking or trash talking." Unless thoughtful leaders create a safe environment and cultivate a positive mindset teachers can have all the data in the world, but they won't be able to use the information to guide improvement.