How do they do accurate twin studies if twins are rare?

Good question!: Even though twins/triplets have become more common, we do need a lot more good studies performed so that we can answer important questions! the problem is usually that we don't have enough study participants to get a good sample size in order to answer a study question. Studies might not be accurate for a host of other reasons, but adequate participant numbers does help!

Depends: Depends on the type of study as there are many types of epidemiology study types