# Resolved: How can I know if a one-sided test is left-tailed or right-tailed? Please help

5.7.+Test+for+the+mean.+Dependent+samples_exercise

"Some guy on the internet designed a weight-loss program. You are wondering if it is working. You are given a sample of some people who did the program. You can find the data in kg if you prefer working with kg as a unit of measurement." This is what the exercise is asking

The Quick P-value calculator provided by 365 Datascience asks for the type of the hypothesis test. I knew how to differentiate between one-sided and two-sided tests, but I dont know how to determine the left-tailed and right-tailed one. Please help me with this asap, thank you!

Hi Huyen!

Thanks for reaching out!

To determine if a one-sided test is left-tailed or right-tailed, look at the alternative hypothesis. A right-tailed test is used if the alternative hypothesis states that the parameter is *greater *than a certain value, placing the critical region in the right tail. Conversely, a left-tailed test is used if the alternative hypothesis states that the parameter is *less *than a certain value, placing the critical region in the left tail.

For the given situation, where you want to determine if the weight-loss program is effective, you need to compare the weights of individuals before and after the program. This typically involves testing if there is a significant *decrease *in weight after the program. Therefore, this would be a left-tailed test because you are testing if the mean weight after the program is *less *than the mean weight before the program (stated in the alternative hypothesis).

Hope this helps.

Best,

Ivan