The whole Neptini is a difficult group and many species are alike.

This is a taxonomically difficult group of very similar species.

But, as they have discovered over the past year, tied agents can be expensive and difficult groups of people to do business with.

"They're a difficult group to get to the set consistently," he said.

This was considered to be the most difficult group to advance from in the tournament.

"I expected us to be drawn into a difficult group."

This is the last upgrade of a very difficult group in a region near the Gap.

"We are in the most difficult group, no doubt about that at all."

This is a notoriously difficult group to identify at sea.

Suppose, though, that we wanted a sample of a rather more difficult group.