民主社會的意義在於能真正的實現主權在民，人民有權利選擇政黨以及執政者，因此能支持民主核心是完善的選舉制度。台灣的民主體現在政黨的多元及相互制衡與競爭，在這政黨激烈競爭下，中間選民變得格外重要，中間選民是我們無法透過民調發現的未知數，這些未知數往往成為左右選舉結果的重要關鍵。我們使用民調中的未表態選民替代中間選民進行電腦模擬，觀察未表態選民的多少對投票的影響， 2012台灣總統大選預測市場偏誤的原因以及投票模型中預測市場的影響。 結果顯示未表態選民越多對投票的影響越明顯，2012台灣總統大選預測市場偏誤是因為選情激烈所造成，預測市場在投票模型佔有重要的地位，在資訊不足時可能會加大誤差，但當資訊收集到一定程度時，預測市場將具有修正投票結果的能力。 The essence of democracy is sovereignty of the people. People elect the political party, the capable administrator, and the law, so the democracy is demonstrated by the election system. Party competition is a distinguishing characteristic in most of the nations including Taiwan. In this aggressive competition of political parties, the median voters become important. Although median voters are unknown, the election results are often crucially dependent on these voters. We apply the idea of "undecided voter" to model the "median voter" in our computational simulation. It is observed that the undecided voters significantly influence the result of the prediction market of the 2012 Taiwan Presidential election event. The simulation results showed that the increasing of the proportion of undecided voters will enhancing the distortion of the voting results, especially in aggressive competition like 2012 Taiwan presidential election. The predict market claims an important position in the voting model. Prediction market provides aggregation information which should improve the revelation of the real opinion of the people.