Selective sample:样本选择，一部分被排除在外

One major problem is about the sample the author studied in the survey. the author considers only...(selective sub-groups) although (the sub-groups the author studied) do constitute a significant part of (the whole general population), however, (many other sub-groups) are actually excluded from the survey. Perhaps...or...without ruling out all the possibilities above, the author could not make any general conclusion about...merely based on (selective sub-groups).