Survey research as a method of collecting sample data is supposed to produce sample statistics which can estimate the corresponding population parameters if the sampling design is appropriate. However, for reasons such as unit non-response, survey data is usually weighted by the institutes that collect the data or by researchers who analyse the data in order to correct or diminish the discrepancies between sample and population. Sample statis-tics based on weighted data are more representative of the population para-meters than unweighted data in terms of some demographic characteristics. Therefore, to some extent, it seems legitimate to weight data and this ma-nipulation has become a routine when dealing with survey data. It is true that to weight data could be helpful, but this manipulation ne-eds justifications. This paper therefore tries to argue that to weight data is no panacea and should not be taken for granted when considering the exam-ples in Taiwan’s Election and Democratization Studies (TEDS) surveys. The first section discusses why weighted data is not necessarily representative of the population. As the TEDS surveys show, the turnout, the vote shares of parties, and marital status become more deviant from the population para-meters after weighting the data.