With the growing emphasis on the development of scientific inquiry skills, there is a strong need for more research on students' ability to collect and interpret evidence. This paper calls attention to the notion of evidentiary competence that refers to the concepts and reasoning skills involved in the collection, organization, and interpretation of data. We proposed a set of concepts and skills involved in evidentiary competence and examined sixth of them--the priority, relevancy, objectivity, replicability of evidence, and the interpretation of examples and tables--using a written instrument contextualized in atmospheric science. Analyses of 40 sixth grade students' answers and explanations revealed that their understanding of scientific evidence and the data collection process was quite weak in several respects. For example, many students neither appreciated the role of empirical evidence in scientific inquiry, nor distinguished relevant evidence from irrelevant evidence, nor understood the importance of reliable and objective observations, nor interpreted examples and tables appropriately. Results suggest that more explicit instructions are needed in order to strengthen students' ability to collect and interpret data, especially in the current data rich information age.