Den här rapporten har diskuterat problemet med hypotetisk bias i CV-studier, d.v.s. det faktum att individer ofta tenderar att överskatta betalningsviljan för en vara i enkätstudier, jämfört med vad de skulle betala i riktiga situationer. Detta leder till stora problem när man skall beräkna ekonomiska värden av icke-marknadsvaror, såsom miljövärden. En genomgång av litteraturen visar att det finns stark evidens för att hypotetisk bias är ett vanligt och omfattande problem. Den genomsnittliga kalibreringsfaktorn låg på 3.4-3.7 (överskattningen av värdet vid hypotetiska studi- er) medan medianen av kalibreringsfaktorn låg kring 1.7. Att genomsnittet är avse- värt högre än medianen beror på ett fåtal studier som visar extremt hög hypotetisk bias. Ett problem med dessa experimentella studier är att de sällan varit utformade enligt de rekommendationer som finns för välstrukturerade CV-studier som ska genomföras i stor skala. Snarare har de experimentella studierna kring hypotetisk bias ofta varit konstruerade i liten skala med fokus på att besvara metodfrågor. Detta gör att den externa validiteten är tveksam. Trots detta utmynnade min re- kommendation i att Naturvårdsverket och andra myndigheter som använder sig av ekonomiska värderingsstudier (CV-baserade) skall justera dessa värden neråt för känslighetsanalyser. Som ett exempel, om en åtgärd för att sanera en övergödd sjö utreds med en samhällsekonomisk utvärdering och CV-studier beräknar ett värde på saneringen, bör detta värde i känslighetsanalyser skrivas ner med kalibrerings- faktor 3. Visar åtgärden lönsamhet även med dessa nedskrivningar kan man känna sig komfortabel med åtgärdens samhällsekonomiska effektivitet. Jämför man två olika projekts lönsamhet sinsemellan är detta mindre nödvändigt, då hypotetisk bias verkar vara ungefär lika stort problem oavsett typ av vara (möjligt undantag för starkt ”etiska” varor”, som leder till högra bias).
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