Effect size for Wilcoxon Signed Rank

lynnar

New Member
1) i want to calculate the effect size for wilcoxon signed rank (paired sample). I have one group of 10 participants.

I know the formula for the effect size is Z/squaroot of N.

My question is, does N refer to the numbe of participant (N=10) or should I multiply the number of participant with 2..so N = 20?

For mann whitney U, i should add the number of participants in both group, but I am not sure should i multiply the number of participant in wilcoxon.

2) DO i need to calculate the effect size for all conditions? or only thos with significant diffetence (P<.05)

3) DO i ignore the 'negative' sign when reporting the effect size?

Thanks!

Karabiner

TS Contributor
i want to calculate the effect size for wilcoxon signed rank (paired sample).
No. You want to calculate an effect size MEASURE in order to describe your sample data.
True effect sizes belong to populations. What you have in your sample is "contaminated"
to a large amout by chance variation. That's the reason why tests of signifcance are
performed instead of simpy reporting effect size measures.

Anyway,
My question is, does N refer to the numbe of participant (N=10)
Yes.
2) DO i need to calculate the effect size for all conditions? or only thos with significant diffetence (P<.05)
It depends. Within one single study, effect sizes measures are more or less meaningless,
in my humble opinion, as long as sample sizes are small. But readers might like to compare
these measures across studies. So it may be nice to report all of them.
3) DO i ignore the 'negative' sign when reporting the effect size?
I would say no, since if one wants to compare across studies, the sign is of course
very important.

Kind regards

K.

lynnar

New Member
if my sample size is 10 or 19 (in two different studies), is it too small to calculate the effect size? and the significant difference?

Karabiner

TS Contributor
You can always calculate an effect size measure. I do not know what you mean by
the second question, but you can calculate p-values, and you can calculate which
sample difference is necessary to obtain a statistically significant result.

Kind regards

K.

New Member
However I came across one publication (abstract and introduction only, as i am not able to access it for free) which suggests that effect sizes are to be reported for all primary outcomes inclusive of the insignificant ones.
Check Hill and Thompson (2005). Higher Education: Handbook of Theory and Research... Oh sorry I dont have the complete citation. But I hope this will help.

New Member
Does anyone know how to culculate the effect size for multiple response data? The effect sizes for 2x2 and ordinal data assume that the data is mutually exclusive (1 response per case). My data is such that respondents were asked to tick as many as applied to them, which violates the mutual exculisve assumption. Please help.

lynnar

New Member
HI..i'm sorry i have to ask again..

for wilcoxon, it is a 2 dependent sample..so when I calculate the effect size ( z divided by square root of N) ..does the N refer to number of sample or number of test condition?

in my case I have 10 participants, so does it mean N= 10 or N = 20 (10 x 2 is the total number of condition)//

Thanks!

lynnar

New Member
i wanted to ask something related to wilcoxon reporting...is it necessray to report the effect size R for APA style reporting? or is it enough just to report only the median, Z and -value?

I've seen a lot of reporting regarding cohens d effect size (t test), but not really much on effect size r (or do i miss anything?)

Can anyone help?

Thanks!!!!