Solved – Wilcoxon one-tailed signed rank test

wilcoxon-signed-rank

It's regarding a one-tailed Wilcoxon sign test.

I am comparing (fictional) ratings of liking before and after an apple is peeled.

The design is complicated. I will do my best to explain.

There are 9 apples. Participants are asked to rate how much they like each apple. After that they are asked if the apple should be peeled. Those who answered yes, are asked to rate how much they like the apple after it was peeled while those who answer no move to rating the next apple. (please remember this is fictional, my actual research is diff. but I don't want to post it here so using a fictional research which is nearly similar).

So basically, I have pre and post peeling ratings.

Participant one may have 5 pre and post ratings while part. 2 has 7 pre and post ratings.

I expect pre peeling ratings to be more than post peeling ratings, hence one-tailed.

This is the output from SPSS:

Descriptive Statistics
N Mean Std. Deviation Minimum Maximum Percentiles
25th 50th (Median) 75th
rating1 379 4.000 1.08588 1.00 5.00 3.0000 4.0000 5.0000
rating2 379 3.500 1.70245 1.00 5.00 3.0000 4.0000 5.0000

Ranks
N Mean Rank Sum of Ranks
rating2 – rating1 Negative Ranks 135a 63.00 7975.00
Positive Ranks 0b .00 .00
Ties 254c
Total 389
a rating2 < rating1
b rating2 > rating1
c rating2 = rating1

Test Statisticsa 

                           rating2 - rating1
           Z              -10.331b
    Asymp. Sig. (2-tailed)     .000

a Wilcoxon Signed Ranks Test
b Based on positive ranks.

As far as I can see, the difference is signficant p<0.001 (two-tailed).

How about one-tailed? How do I know if it is signficant at one-tailed as my hypothesis is one-tailed…I have predicted that post peeling ratings will be less than pre-peeling.

Best Answer

You can get the one-tailed p-value just by dividing in half the two-tailed p-value. But keep in mind that it's generally not advisable to use one-tailed tests.

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