# Thread: can non parametric tests be modified for one tailed tests?

1. ## can non parametric tests be modified for one tailed tests?

Hello all,
super newbie here.
just a quick question:
I have been doing one tailed independent two sample t tests but now on other data I no longer have normal distributions so I wanted to use Mann Whitney U test, but I am not sure if these can be one tailed.
I am using R so is it legitimate to specify the "alternative" argument as greater and if so (Im assuming so since it is an available option in R) do I then divide p/2 as I would for the t test to get a one tailed p value?
cheers
J

2. ## Re: can non parametric tests be modified for one tailed tests?

I have been doing one tailed independent two sample t tests
Normally, you should not perform one-tailed tests.
You make yourself half blind.
but now on other data I no longer have normal distributions so I wanted to use Mann Whitney U test,
If your total sample size is > 50 or so, non-normal
distributions shouldn't matter much. One has to be
more concerened with unequal variances (if samples
have unequal size).
do I then divide p/2 as I would for the t test to get a one tailed p value?
Seems so. But normally you shouldn't do so.

With kind regards

K.

3. ## Re: can non parametric tests be modified for one tailed tests?

Thanks for that.

My samples are fairly small. I'm testing the direction hence the one tail.

Cheers
J

4. ## Re: can non parametric tests be modified for one tailed tests?

I'm testing the direction hence the one tail.
Many people test the direction, but you hardly ever
see one-tailed tests. Cleary, the usual two-tailed
test is absolutely ok for testing directional working
hypotheses. Just having in mind what the direction
of an effect might be does not justify one-tailed testing,
because, as mentioned before, it makes the researcher
half blind without good reason.

With kind regards

K.

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