Two Tailed T Test And Directional Hypothesis
In the case of a nondirectional null hypothesis, a twotailed test of significance is used.
Chapter 11 Two tailed t test and directional hypothesis
If the conclusion were to support the claim that the antibiotic prolonged group A life spans, then the researchers should use a directional alternate hypothesis, such as Ha: XbarA > XbarB. Here group A's life span is hypothesized to be greater (longer) than group B's (the control group). In this case, an alpha level of 0.05 implies that all 0.05 would have to appear in the right or high tail of the curve, which then is a onetailed or directional test, as shown in Figure 83. This figure shows that the critical tvalue will actually be smaller for the onetail test, that is, +1.65 instead of 1.96 or 2.00 from the twotail test. This happens because 95% of the area under the curve begins to accumulate from the leftmost side of the curve (including that tail) and includes less of the right side of the curve. The result is that t_{calc} can be smaller (1.65 instead of 1.96) and still cause Ho to be rejected.
Now don't get confused  we're not testing to see if our mice have two tails! We're testing to see if the mean of the sample group is either less than or greater than the mean of the control group, which  in statistical terms  is considered to be a twodirection or twotailed test. Remember that the hypotheses were Ho:XbarA = XbarB and Ha: XbarA is not equal to XbarB. In this alternate hypothesis, all that has been said is that the two means are not the same, which would be true (a) if the mean of the sample group is higher than that of the control group or (b) if the mean of the sample group is lower than that of the control group. There is nothing in the phrasing of the hypothesis that stipulates the group A animals (treated) must actually have longer life spans as compared to the group B animals.
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Remember the example of testing the effect of antibiotics on mice in Lesson 7. The point of the study was to find out if the mice who were treated with the antibiotic would outlive those who were not treated (i.e., the control group). Are you surprised that the researcher did not hypothesize that the control group might outlive the treatment group? Would it make any difference in how the hypothesis testing were carried out? These questions raise the issue of directional testing, or onetailed vs twotailed tests.
OneTailed Vs. TwoTailed Tests: Differences & …
10/01/2018 · Directional Hypotheses ..

Experimental Questions and Hypotheses  Missouri S&T
A twosided hypothesis is used when there is no strong directional hypothesis expected in a research study

let's take two fairly simple ..
A researcher hypothesis directional may use a twotailed test to evaluate a directional hypothesis

directional hypothesis explanation free
Hypothesis  Wikipedia