A recent article published in People (2021) states that a study "which has not yet been peer-reviewed, studied the relationship between self-reported alcohol consumption in about 25,000 U.K. residents and the amount of gray matter in their brain scans. Researchers determined that alcohol consumption "was linearly negatively associated with global brain gray matter volume" meaning that even small amounts of alcohol intake correlated to less gray matter." Based on this information, what type of study was conducted