Two-tailed tests are appropriate for most studies.
A non-directional hypothesis states that a population parameter (such as a mean or regression coefficient) is not equal to a certain value (such as 0). 
Two-tailed tests are used when the alternative hypothesis is non-directional. To know whether to reject the null hypothesis, you need to compare this t value to the critical value of t. You calculate the t value for the sample.
Alternative hypothesis: The treatment group and control group participants differ in their mean numbers of pimples. Null hypothesis: The treatment group and control group participants have the same mean number of pimples. To discover whether the acne cream is effective, you decide to compare the mean number of pimples on participants in the treatment and control groups using an independent samples t test. You randomize the participants into a treatment group that receives the acne cream and a control group that receives an unmedicated placebo cream. Example: A t test case studyImagine you’re conducting a small trial for a new medicated acne cream. If you need to find a critical value of t to perform a statistical test or calculate a confidence interval, follow this step-by-step guide. The critical values of t are difficult to calculate by hand, which is why most people use a t table or computer software instead. Student’s t distribution is the distribution of the test statistic t. The critical values of t are calculated from Student’s t distribution. Calculating confidence intervals (of means or regression coefficients). Testing whether two variables are significantly related ( linear regression or correlation). Testing whether two means are significantly different (two-sample t tests). Student’s t table is also known as the t table, t-distribution table, t-score table, t-value table, or t-test table.Ī critical value of t defines the threshold for significance for certain statistical tests and the upper and lower bounds of confidence intervals for certain estimates. Student’s t table is a reference table that lists critical values of t. Try for free Student's t Table (Free Download) | Guide & Examples 
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