[R] Wilcox paired test error message

[peter dalgaard]

On Jun 23, 2013, at 22:30 , Sarah Goslee wrote:

> Hi,
> 
> If you're doing exactly as described below, then you need to add the
> data argument to wilcox.test so R knows where to find beforesmall and
> aftersmall. But if so, that's a rather uninformative error message.
> 
> Saraj

Not really, if you use the formula interface, the rhs is supposed to be the grouping, as in 

wilcox.test(extra ~ group, data=sleep, paired=TRUE)

so it's telling you that "aftersmall" does not describe two groups.

(It is unfortunate, though, that we don't have a formula interface to the parallel-vector data layout for paired tests. That is the common case, the sleep data set is a rather rare exception. A formula specification for paired columns has been talked about; something like cbind(before, after) ~ 1 should be workable, but nothing has materialized to date.)

-pd   

> 
> On Sun, Jun 23, 2013 at 2:34 PM, ruth parker <parkeractonruth at gmail.com> wrote:
>> Hi,
>> I've been trying to run a wilcox paired test on some data
>> beforesmall aftersmall
>> [1,]        63.5      512.0
>> [2,]        54.5      237.5
>> [3,]        52.5      161.5
>> [4,]        78.0      153.5
>> [5,]        53.5       68.0
>> [6,]        50.5       65.5
>> [7,]        69.0       52.0
>> [8,]        76.0       59.0
>> [9,]        68.0       66.5
>> [10,]        75.5       66.5
>> [11,]        67.0       45.5
>> [12,]        81.0       54.5
>> [13,]        49.0       44.0
>> [14,]        51.0       42.5
>> [15,]        53.0       34.5
>> 
>> using
>> wilcox.test(beforesmall ~ aftersmall,paired=T)
>> but I get the error message:
>> 
>> Error in wilcox.test.formula(beforesmall ~ aftersmall, paired = T) :
>>  grouping factor must have exactly 2 levels
>> 
>> I don't have any missing values, I have the same amount of data points for
>> each group and I've looked everywhere trying to find the answer.
>> Please can somebody tell me how to make it work
>> thanks
>> 
> 
> -- 
> Sarah Goslee
> http://www.functionaldiversity.org
> 
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-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com