Thankfully SIDS is very rare, and doing studies with small numbers is notoriously difficult.
There are so many variables, birth weight, feeding method, sleeping place, parents ages, babies age, families social economic status, smoking, drugs and drink. Was the baby ill?
Data to be collected either from parents who are going to be utterly heart broken or long after when details are blurred by time and grief. Else information dug out of previous studies, hospital and police records where slightly different questions will have been asked.
It's a researchers nightmare.