Is it possible to achieve some of the benefits of formal testing within the informal programming conventions of the spreadsheet paradigm? We have been working on an approach that attempts to do so via the development of a testing methodology for this paradigm. Our "What You See Is What You Test" (WYSIWYT) methodology supplements the convention by which spreadsheets provide automatic immediate visual feedback about values by providing automatic immediate visual feedback about "testedness". In previous work we described this methodology; in this paper, we present empirical data about the methodology's effectiveness. Our results show that the use of the methodology was associated with significant improvement in testing effectiveness and efficiency, even with no training on the theory of testing or test adequacy that the model implements. These results may be due at least in part to the fact that use of the methodology was associated with a significant reduction in overconfidence.