One Tailed  and Two  Tailed Tests

The critical  region ( or the region of rejection)  which is generally 5 per cent is kept  on both  sides of the normal  distribution in a two  tailed test. It means that 2.5  per cent  of the critical  is on the extreme left of the normal curve and 2.5 percent on the extreme right. The middle 95 % is the acceptance  region in a single tail  test the 5  per cent  area   would  be either  on the  extreme  left  of the normal  curve or on the extreme right. The  remaining  95 percent  area would  be the  acceptance region.

Two tailed test is applied  when the  null  hypothesis  is that  mean  weight of the  undergraduate  male  students is 120 pounds  and the  alternate  hypothesis  is that it is not 120 pounds. In such  a situation the  actual  value  of the population  mean or maybe more  than  120 pounds or less than  120 pounds. In such  situation a two  tailed  is appropriate.