<P> In statistical significance testing , a one - tailed test and a two - tailed test are alternative ways of computing the statistical significance of a parameter inferred from a data set , in terms of a test statistic . A two - tailed test is appropriate if the estimated value may be more than or less than the reference value , for example , whether a test taker may score above or below the historical average . A one - tailed test is appropriate if the estimated value may depart from the reference value in only one direction , for example , whether a machine produces more than one - percent defective products . Alternative names are one - sided and two - sided tests ; the terminology `` tail '' is used because the extreme portions of distributions , where observations lead to rejection of the null hypothesis , are small and often `` tail off '' toward zero as in the normal distribution or `` bell curve '' , pictured on the right . </P>
how do you know if you have a two tailed test
if the estimated value may be more than or less than the reference value