Case–control studies compare a group of patients who experience an event with another not experiencing the same event, and examine how exposure to a suspected agent (for example, therapeutic intervention) differs between groups.
This type of study design is useful to ascertain the cause of rare events, although provides weaker evidence than cohort studies.
The statistical method used should adjust the comparisons for all potential prognostic factors, and the causality between treatment and observed difference should be discussed.
Despite international variation in health-care provision, all systems have finite and constrained resources and delivery of ‘high-cost, low-volume’ procedures, such as HSCT, is a public health challenge.
Progress in the field of HSCT in ADs has been limited by funding not only of clinical trials, but also of individual HSCT procedures irrespective of the health-care provider.