One Tail Two Tails

Six Sigma – iSixSigma Forums Old Forums General One Tail Two Tails

This topic contains 2 replies, has 3 voices, and was last updated by  Bill Craig 13 years, 9 months ago.

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    How to pick up the right P value for interpreting two sample or paired t-tests? How to know whether the one tail or the two tails value is to be considered?


    Robert Butler

     If detecting a significant difference in either direction is important use the two tailed test. If your only concern is detection of a difference in one direction then use the one tailed test.
    Two Tailed – I have a drug and a proposed replacement. I care about knowing if there is an improvement but it is also important for me to know if there is a decrease in the effectiveness of the new drug vs. the old one.
    One Tailed – I”ve got a new machine and all I care about is will it do the job faster than the old one


    Bill Craig

    Make sure you construct your null and alternate hypotheses before you do your experiment, and the selection of the correct p-value will be clear. Robert suggested some good examples.
    State Ho, Ha, and the decision rule up front. (ie reject Ho if t-calculated is greater than t-critical.)

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