[R] Wilcox paired test error message

[David Winsemius]

On Jun 23, 2013, at 11:34 AM, ruth parker 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

In this instance, you are working with a matrix, so you need supply the two column vectors explicitly:

----------

wilcox.test( dat[ ,'beforesmall'] , dat[ , 'aftersmall'], paired=T)

------------

And if you are using attach() and haven't told us, then STOP DOING THAT. (It won't work with matrices and it creates confusion among the inexperienced people who use it most often.)
-- 


> 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
> 
> 	[[alternative HTML version deleted]]
-- 

David Winsemius
Alameda, CA, USA