There have been warnings of practical difficulties of no - notice inspections, such as the head teacher being away that day or pupils being out on school trips. (bbc.co.uk)
In addition to sampling error, one should bear in mind that question wording and practical difficulties in conducting surveys can introduce error or bias into the findings of opinion polls. (monmouth.edu)
I understand both the initial shock of being told «you can not eat this food anymore» and also the social, economical and practical difficulties with implementing such a restriction. (nutritiousndelicious.com)