Size of the sample for ranksum analysis

I am using ranksum with two samples of 200 elements and I get a p value which does not make any sense and if I do the boxplot I can clearly see that there is no difference between the two samples even though I get h=1 and a p value really small. Is there any other function I should use for this prupose?

[Jos (10584)]

Without knowledge of your values or code, it is impossible to tell if you applied ranksum correctly. Note that even small, unimportant, differences can become statistically significant when you increase the N ...

Answer 1

[Blanca Larraga]

Well, I have an excel file with the data. I import the data with the xlsread function to get to column of values from this file. I declare x1 with the 200 data with one condition and then I declare y1 with the other 200 data which are similar/different and I want to see if these two samples are related or not. My code is simple:

x1=xlsread('datos_conjunto.xlsx', 'M_paras_i_3', 'A2:A202');

y1=xlsread('datos_conjunto.xlsx', 'M_paras_o_3', 'A2:A202');

[p1,h1,stats1]=ranksum(x1,y1);

If I then use boxplot:

A=[x1,y1]

boxplot(A)

I see from the boxplot that there are no significance but the p-value is something 10^-27 and this does not make any sense.

I hope this clarifies the issue.

Thanks.

Blanca