The impact of computer aided interview on data quality has been systematically evaluated by statistical institutes in diversecountries. This work aims to show the importance of field observation in the evaluation of computer assisted data collection.The method shown here was proposed to be applied to the context of IBGE, the Institute of Brazil central administration thatperforms the Census. This method (Scenario and Task Based Interviews - STBI) was designed to be implemented with theparticipation of interviewers who use personal digital assistants to perform statistical data collection for demographicresearch. Authors analyzed the usability of the application developed to support Continuous National Household SampleSurvey (Continuous PNAD). The proposed method represented a mix of four approaches to evaluation. This work gives aspecial emphasis to describe the part of the method that concerns the application of ethnographic observations and itsconclusions.