What is the difference between a left and right tailed test

The following examples show how to identify left-tailed and right-tailed tests in practice. However, one inspector believes the true average weight is less than 20 grams. To test this, he weighs a simple random sample of 20 widgets and obtains the following information:. He then performs a hypothesis test using the following null and alternative hypotheses:. Since the test statistic is not less than this value, the inspector fails to reject the null hypothesis.

The null hypothesis must always include the concept of equality, which means that it must include the operators equal, less or equal, or greater or equal. On the other hand, the alternative hypothesis must always express the opposite of the operator used for the null hypothesis, which means it should always include not equal, greater than, or less than.

It must always include equal operator , less than operator , or greater or equal operator. If the null expresses equal operator , the alternative expresses not equal operator. If the null expresses less or equal, the alternative expresses greater than. Hypothesis Directional Non-directional Region of rejection Either left or right Both left and right Determines If there is a relationship between variables in single direction.

If there is a relationship between variables in either direction. Result Greater or less than certain value. Greater or less than certain range of values. One-tailed test alludes to the significance test in which the region of rejection appears on one end of the sampling distribution. It represents that the estimated test parameter is greater or less than the critical value. When the sample tested falls in the region of rejection, i.

It is primarily applied in chi-square distribution; that ascertains the goodness of fit. One-tailed test can be:. The two-tailed test is described as a hypothesis test, in which the region of rejection or say the critical area is on both the ends of the normal distribution. It determines whether the sample tested falls within or outside a certain range of values. This means we have to use a two-tailed test. The manufacturer now decides that it is only interested whether the mean lifetime of an energy-saving light bulb is less than 60 days.

What changes would you make from Example 1? This means that instead of performing a two-tailed test, we will perform a left-sided one-tailed test.