Field surveys (or survey instruments/questionnaires) are the primary instrument used by researchers for the process of primary data collection. When secondary sources of data are either insufficient, or not available, field surveys allow researchers to monitor and evaluate the impact of field experiments. For example, consider a study that aims to evaluate the impact of micro-loans on farm output in a small village in Malawi. It is possible that data on farm output for the last 10 years is not available, or is insufficient. In this case, researchers can conduct a survey among local farmers to collect data on farmer incomes and farm outputs.