4. Features Must be Implemented. A live controlled experiment needs to expose some users to a Treatment different than the current site (Control). The feature may be a prototype that is being tested against a small portion, or may not cover all edge cases (e.g., the experiment may intentionally exclude 20% of browser types that would require significant testing). Nonetheless, the feature must be implemented and be of sufficient quality to expose users to it. Jacob Nielsen (34) correctly points out that paper prototyping can be used for qualitative feedback and quick refinements of designs in early stages. We agree and recommend that such techniques complement controlled experiments.