Statistics for Economic non-market valuation techniques : theory and application to ecosystems and ecosystem services : a case study of the restoration and preservation of Pekapeka Swamp : an application of the contingent valuation method in measuring the economic value of restoring and preserving ecosytem services in an impaired wetland : a thesis submitted in partial fulfillment of the requirements for the degree of Master of Philosophy in Economics at Massey University, Palmerston North, New Zealand