R help - Mar 2015 - Using choiceDes Package to Design MaxDiff?

I?m seeking to design a MaxDiff experiment that will have a number of blocks of
this type:

Which of these items is the
most important?

Which of these items is the 
least important?

Item 1
Item 2
Item 3
Item 4

I?m seeking to use the choiceDes package
<http://cran.r-project.org/web/packages/choiceDes/choiceDes.pdf> to design
the experiment. The relevant function is tradeoff.des. Usage:

tradeoff.des(items, shown, vers, tasks, fname=NULL, Rd=20, Rc=NULL, print=TRUE)

I believe I understand the items, shown and vers parameters:
items: number of total items in the experiment
show: number of items shown per block
vers: number of blocks
...but I?m not quite sure what the tasks parameter is yet. For example, let?s
say I have 20 items total in the study. I want to show 4 items per block, with
10 blocks total. I enter:

tempDes <- tradeoff.des(20, 4, 10, 2, "tempDesign.txt", 20, NULL,
TRUE)

?hoping that the 2 is the number of questions (most important/least important)
per block. I get an error:

Error in optBlock(~., des.d, rep(tasks, vers), nRepeats = Rd) : 
  The number of withinData rows is not large enough to support the blocked
model.

If I put the number of blocks up to 50:

 tempDes <- tradeoff.des(20, 4, 50, 2, "tempDesign.txt", 20, NULL,
TRUE)

?I get the same error. 

What is the correct way to use choiceDes to design a MaxDiff experiment of this
kind?

Thanks very much in advance to all for any thoughts or info!

Best,


-Vik



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