In research, how the exclusion of unlisted companies will have impacted on nature of their research sample?

Question

In research, how the exclusion of unlisted companies will have impacted on nature of their research sample?

Answer 1

The research sample needs to be a representative sample of the target population and if the target population is a set amount of companies then it's best to explain why some are excluded from the list etc.

Answer 2

It will result in an upward survivorship bias.   Say, for example, that you're calculating historical stock returns over the past twenty years.  Over time, a number of companies' stock will have been delisted for a variety of reasons, including bankruptcy.  The exclusion of those stocks from your overall index will result in overstated returns.