"Good. We need you."
"Who, me?" asked Sandra.
"Yes."
"Why?"
"Because you are alien. You are impartially alien. Though you have friends on Telfu, they are few, and in your secret mind you class us all as 'Telfan' and forget about sub-classifications. This experimentation is just that, to you, and we are the subjects. Therefore when you select one hundred victims out of a district, we get a perfect, impartial selection; a true cross-section of the district."
"Any of you could do that."
"No. We'd be biased by our knowledge of who is important, who is the sicker, who is young and who is old. And, though it may seem strange to you, you have absolutely no idea of beauty. Therefore you are impartial among the ugly and the beautiful."
"So what?"
"In experimentation on humans, we are inclined to pick those of less value to the community. We pick the lame and the halt and the ugly. We are inclined to pick those who are likely not to live anyway, and this biases our selection. Come, let's get going."
"O.K. Lead on."