what is a two tailed test 
a two tailed test in statistics is a method in which the critical area of a distribution is two sided and tests whether a sample is greater than or less than a certain range of values it is used in null hypothesis testing and testing for statistical significance if the sample being tested falls into either of the critical areas the alternative hypothesis is accepted instead of the null hypothesis 1understanding a two tailed testa basic concept of inferential statistics is hypothesis testing which determines whether a claim is true or not given a population parameter a hypothesis test that is designed to show whether the mean of a sample is significantly greater than and significantly less than the mean of a population is referred to as a two tailed test the two tailed test gets its name from testing the area under both tails of a normal distribution although the test can be used in other non normal distributions a two tailed test is designed to examine both sides of a specified data range as designated by the probability distribution involved the probability distribution should represent the likelihood of a specified outcome based on predetermined standards this requires the setting of a limit designating the highest or upper and lowest or lower accepted variable values included within the range any data point that exists above the upper limit or below the lower limit is considered out of the acceptance range and in an area referred to as the rejection range there is no inherent standard about the number of data points that must exist within the acceptance range in instances where precision is required such as in the creation of pharmaceutical drugs a rejection rate of 0 001 or less may be instituted in instances where precision is less critical such as the number of food items in a product bag a rejection rate of 5 may be appropriate special considerationsa two tailed test can also be used practically during certain production activities in a firm such as with the production and packaging of candy at a particular facility if the production facility designates 50 candies per bag as its goal with an acceptable distribution of 45 to 55 candies any bag found with an amount below 45 or above 55 is considered within the rejection range to confirm the packaging mechanisms are properly calibrated to meet the expected output random sampling may be taken to confirm accuracy a simple random sample takes a small random portion of the entire population to represent the entire data set where each member has an equal probability of being chosen for the packaging mechanisms to be considered accurate an average of 50 candies per bag with an appropriate distribution is desired additionally the number of bags that fall within the rejection range needs to fall within the probability distribution limit considered acceptable as an error rate here the null hypothesis would be that the mean is 50 while the alternate hypothesis would be that it is not 50 if after conducting the two tailed test the z score falls in the rejection region meaning that the deviation is too far from the desired mean then adjustments to the facility or associated equipment may be required to correct the error regular use of two tailed testing methods can help ensure production stays within limits over the long term be careful to note if a statistical test is one or two tailed as this will greatly influence a model s interpretation two tailed vs one tailed test