Reseаrchers gаthered infоrmаtiоn оn the prevalence of adverse childhood experiences (ACEs) in children under the age of six years from a randomly selected sample from a large nation. One type of data collected involved parents reporting about special health care needs oft heir children such as whether they had emotional, behavioral or developmental delays. Children with no or low ACEs were less likely to experience developmental difficulties. Which of the following research methodologies as used in this study?