There have been increasing criticisms of the ability of Nigerian valuers to undertake investment valuations in a reliable and consistent manner. Prior empirical studies that have tended to investigate this claim have however been criticized as the valuers employed in simulated valuations of recently sold properties were not paid and did not inspect the properties. Other studies have been accused of using forced sale values. This study sought to examine whether valuers who carried out fully paid and fully inspected open market valuation assignments were able to do so in a reliable and consistent manner, based on both regression and mean deviation tests. To achieve this aim, the paper employed secondary data of the 131 Federal Government privatised properties which were valued by Estate surveyors and valuers before being sold. Data were analysed with the use of mean deviation and regression analysis. The results confirm that even where property valuations are fully paid for and fully inspected and even where they do not involve forced sale values, they do not yet meet regression based and deviation based standards of reliability. The study concluded that there is the need for the valuation profession to enshrine a maximum acceptable margin of error of ±13.16% in the future valuation standards and ensure more rigorous training of valuers with a view to minimising the incidence of inaccuracy of investment valuations in the country.