This study examined the understanding of 48 Brazilians students of 2nd and 5th grades (seven and ten years old) of statistical variability of data in bar charts. A Piaget’s clinical interview was conducted involving four activities of variability: description or explanation of the variability; representation of variability; prediction results from the variability of the data; comparison between data sets. Students showed ease in recognizing endpoints, but did not make predictions based on what they had observed. The representation of the variability was shown to be an important factor in data interpretation. Make comparison between data sets was complex for most students. Therefore, it is necessary to promote interrelationship among different aspects in order to make students reflect on the data and predictions.