Dictionary of Evidence-based Medicine.pdf

Dictionary of Evidence-based Medicine 35
The contingency table shown is a 3 row x 2 column table and is usually
referred to simply as a 3 x 2 table; 2x2 tables are particularly commonly
encountered. Multidimensional contingency tables are widely used in advanced
categorical data analyses but require advanced graphical multivariate
methods for representation on paper.
Contingent valuation
Estimates of willingness to pay for goods for which there is no market can
be obtained using survey methods. Individuals are asked how much they
would be willing to pay to avoid or overcome a certain state (e.g. avoidance
of a particular morbidity). Using such an approach, sometimes referred
to as 'contingent valuation' or expressed preference, the questions
can be either open-ended or binary (Figure 5). In the former, the patient is
asked the maximum amount he or she would be prepared to pay for the
current treatment in a face-to-face or telephone interview or questionnaire.
In order to improve response rate, response aids such as a bidding game
are used. The patient is shown a value which is either rejected or accepted.
The value is then altered until the maximum payment which is acceptable
to the patient is identified. Alternatively, the patient may be shown a range
of payment values from which a choice is made. The open-ended approach
Figure 5 Contingent valuation: The calculation of willingness to pay using the binary
contingent valuation method