Is it possible to achieve some of the benefits of formal testing within the informal programming conventions of the spreadsheet paradigm? We investigate an approach that attempts to do so via the development of a testing methodology for this paradigm. The "What You See Is What You Test" (WYSIWYT) methodology for testing spreadsheets supplements the automatic immediate visual feedback about values with automatic immediate visual feedback about "testedness". In this thesis, we present empirical data about the methodology's effectiveness resulting from two controlled experiments. The first experiment provided interesting but inconclusive results which spurred us to consider ways to improve the design of our experiment. We used the Cognitive Walkthrough method to evaluate and improve our design and readministered the experiment. Our results from the redesigned experiment 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. / Graduation date: 2000