The authors emphasised that causality could not be established from a retrospective study of this design due to the numerous potential biases that may confound the association. For example, it is known that viral respiratory tract infections in infancy such as respiratory syncytial virus (RSV) are associated with an increased risk of asthma in later childhood and that paracetamol use for such episodes could have caused confounding in the study.
作者强调由于存在很多可能混淆这种关联性的潜在偏差，所以无法从对回顾性研究中确定其因果关系。比如，大家都知道婴儿时期的病毒性呼吸道感染，如呼吸道合胞病毒（RSV）会增加该婴儿在儿童时期感染哮喘的可能性；而如果这个婴儿在同一时期也使用了扑热息痛，那么就会使研究产生混淆。