[R] count ties after rank?

[David Winsemius]

On Nov 21, 2011, at 2:50 PM, Peter Langfelder wrote:

> On Mon, Nov 21, 2011 at 11:36 AM, Hao, Zhaozhe <haozhaozhe at ou.edu>  
> wrote:
>> Hello!
>>
>>    I need to use Kruskal-Wallis test and post-hoc test (Dunn's  
>> test) for my data. But when I searched around, I only found this  
>> function: kruskal.test. But nothing for Dunn's test.
>>
>>    So I started to write one myself.  But  I do not know how to  
>> count ties in the data frame. I can use for loops but it seems long  
>> and unnecessary since the rank function actually knows the ties. So  
>> can anyone give me a hint on how I can "count" the number of ties?
>>
>
> If you just need the overall number of ties in a vector (say x), you
> can get it by
>
> nTies = length(x) - length(unique(x))

And if you wnat to know which ties are which you can do:

table(x)[table(x) > 1]

>
> This will not work if missing data are present, so you will have to
> remove those first.
>

table() would ignore missing data (assuming it were properly NA not- 
valued).

 > x <- sample(c(1:10,NA,NA), 20, replace=TRUE)
 > table(x)[table(x) >1]
x
  1  4  7  9 10
  2  3  2  2  2
 > x
  [1]  7 10  3  9 NA NA  2  4 10 NA NA  4  5  9  6  4  1  1  7  8

-- 

David Winsemius, MD
West Hartford, CT