Analisis perbandingan kinerja perusahaan dan abnormal return saham pada perusahaan pengakuisisi sebelum dan sesudah merger dan akuisisi : studi di Bursa Efek Indonesia periode 2000-2007 = Analysis of comparison of corporate performance and abnormal retum on acquirer companies before and after merger and acquisition : studies on the Indonesia Stock Exchange at time line 2000-2007