Your alternative hypothesis is directional if it includes words such as “greater than,” “less than,” “increases,” “decreases,” or the “” sign.A directional hypothesis states that a population parameter is greater than or less than a certain value.One-tailed tests are used when the alternative hypothesis is directional. If you’re calculating a confidence interval, choose two-tailed.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).
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