V(conservative treatment/surgery treatment) =β0+ β1*chance of pain reduction_med + β2* chance of fatigue + β3* chance of pregnancy + β4*presence of endometriosis_med+ β5 *chance on depressed mood_med + β6 * chance on osteoporosis_med+ β7*chance of pain reduction_surg + β8* presence of endometriosis_surg + β9* chance on temporary stoma_surg + β10* permanent intestinal symptoms_surg + β2* chance of fatigue + β3* chance of pregnancy + ε
V represents the relative utility that a respondent derives from choosing conservative treatment or surgery. β0 is the alternative specific constant, reflecting a preference for the label irrespective of the levels of the attributes
β1- β10 are the alternative specific coefficients of each attribute with the exception of β2 chance of fatigue and β3 chance of pregnancy. The levels of these attributes were generic across the treatments.
A priori, we expected that patients prefer a higher chance of pain reduction (positive coefficient), lower levels of fatigue (positive coefficient for decreasing fatigue, negative for increasing fatigue), higher pregnancy chance, reduction of endometriosis nodules/spots, low chance of getting a depressed mood, osteoporosis, temporary stoma and permanent intestinal symptoms.
ε is an unobserved component of the utility function or error term. Pain reduction, chance on depressed mood, chance on osteoporosis, chance on temporary stoma and permanent intestinal symptoms were included as continuous variables while for fatigue, chance on pregnancy and presence of endometriosis dummy coding was used. In addition, we performed subgroup analysis with women with or without a future child wish. Relative importance was calculated by multiplying the coefficient of an attribute with the range used for the attribute levels or using the difference in coefficients between the best and worst level of the same attribute (in case of dummy coding). Subsequently, the resulting part-worth utility of each attribute was divided by the sum of all part-worth utilities which gives the relative importance per attribute (28). A significance level of 5% was chosen to determine statistically significant coefficients.