This projects aims to: (1) establish best practices for application of target trial emulation methodology using UK CF Registry data to estimate effects of treatments in cystic fibrosis. (2) apply the methods to emulate four existing RCTs that have been conducted assessing the effects of antibiotics. (3) extend Aim 2 to questions that have not yet been studied in RCTs, including to estimate longer-term 
effects of antibiotics and the effects in CFTR modulator users.