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For example, how would you decide between which tailed test to do in the following experiment:

Ethanol dilutions were made with different proportions and Daphnias were submerged in them to collect their heart rates. The Daphnias were also submerged into beakers with different temperatures.

My null hypothesis was that the Daphnia will not be affected by the changing variables and if they are, it is due to a purely kinesis response (caused by chance).

My alternative hypothesis was that Ethanol would decrease the heart rates of the Daphnias as the dilution concentrations increase. I assumed this because Ethanol is a respiratory and nervous system depressant that acts as an anesthetic and potentially leads to death after an overdose. I also assumed that the Daphnias' heart rates would fall and become steadier in lower temperatures because they usually inhabit cooler, freshwater habitats.

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    $\begingroup$ The body text of your question is very unclear, but in respect of your title -- it depends on whether your alternative hypothesis is one sided or two sided ... and we can't really tell you what to hypothesize. $\endgroup$
    – Glen_b
    Commented Mar 7, 2015 at 7:24
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    $\begingroup$ I must agree with the first comment by Glen_b. However, even if we cannot tell you what to hypothesize, it could be beneficial for you to learn more about both options. See, for example, here (sections "When is a one-tailed test appropriate?" and "When is a one-tailed test NOT appropriate?"). By the way, your question should be quite popular in general so there is quite likely a lot of material about it. Introductory statistics textbooks should have some discussion on the topic. $\endgroup$ Commented Mar 7, 2015 at 14:58

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If your result was significant in either direction, would this be note-worthy? In other words, if the data shows that Ethanol actually increases, rather than decreases, the heart rates of the Daphnias as the dilution concentrations increases would this be an interesting result worthy of being noted? If so, do a two tail test.

If it is not possible that Ethanol actually increases the heart rates of the Daphnias, based on past research in the field or common knowledge, do a one-tail test. Of course, due to random chance, the data could suggest Ethanol increases the heart rates. In this situation, we can chalk this up to error hence not note-worthy.

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