a natural resource damage assessment to the extent that they can be reliably measured [46]。

The second point concerns mainly some technical problems involved in CV studies,  including:

●Information bias: CV assumes that people under- stand the good in question and will reveal their preferences in the contingent market just as they would in a real market。

●Strategic bias: Inpiduals may provide answers in order to direct the survey at specific results。 Hence, they may state lower or higher prices, expecting particular benefits。

●Hypothetical   bias:   It   is   argued   that   there     is a fundamental difference in the way people make hypothetical decisions in comparison with the way they make actual decisions。 Hence, since respond- ents do not actually pay for the proposed actions,  it is possible to overestimate the amount that they would pay in a real   situation。

●Payment bias: The proposed method of payment influences the respondent’s statement。 For example, some people may value the good, but state that they are not willing to pay for it, because they are protesting some aspect of the scenario, such as increased taxes。

●ParteWhole bias: In many cases where people are first asked for their WTP for one part of an environmental asset (e。g。 a part of a National Park) and then asked to value the whole asset (e。g。 the whole National Park), the amounts stated are found to be similar。 This phenomenon seems to lie in how people allocate their spending [35]。

●WTP vs。 WTA: The payment question can either be phrased as ‘‘What are you willing to pay to receive this environmental asset?’’ or as ‘‘What are you willing to accept in compensation for giving up this environmental asset?’’ [35]。 Although utility theory predicts that for commodities where there  are limited possibilities for substitution WTA could be much greater than WTP [49], it is still a problem。 Given the observed differences in the estimates, one could use WTP or WTA accordingly, in order to approve or to reject the plan in    question。

It should be mentioned, however, that the above- mentioned biases could be reduced to a great extent or even eliminated by a proper survey design。 In addition, CV presents certain advantages。 First, it is very flexible, and can be  used  in  estimating the  economic  value  of a variety of environmental assets。 Second, it is the most widely accepted method for estimating total economic value。 Third, environmental goods and services that are clearly understood by the respondents can easily be estimated, even if there is no observable behaviour available  to deduce values  by other means。  Nowadays,

a great deal of research is being conducted to improve the methodology and to make the results more valid and reliable。

3。4。Revealed  preference methods

3。4。1。Travel Cost Method

The Travel Cost Method (TCM) utilizes expenditure incurred in  getting  to  the  site  under  investigation, as a surrogate for the price paid from an inpidual visitor for the site’s use。 The idea for the TCM is attributed to Hotelling, who proposed the basic notion of the method to a park service director in 1947。 However, it was developed principally in papers by Clawson [50] and Clawson and Knetschm [51]。 The basic premise of the method is that the time and travel cost expenses that people incur to visit a site represent in some way the recreational value of the site。 Using a number of observations,  a  demand  curve  can  be  obtained and the value of the site can be estimated。 The raw materials for  TCM  analyses  are  inpiduals’  travel  costs    and a number of parameters that must also be taken into account。