[R] Need a factor level even though there are no observations

[Prof Brian Ripley]

Set levels not labels in the factor call.  E.g.

> factor("School", levels = factorlabels)

On Sun, 8 May 2005, Ajay Narottam Shah wrote:

> I'm in this situation:
>
>     factorlabels <- c("School", "College", "Beyond")
>
> with data for 8 families:
>
>     education.man  <- c(1,2,1,2,1,2,1,2)       # Note : no "3" values
>     education.wife <- c(1,2,3,1,2,3,1,2)       # 1,2,3 are all present.
>
> My goal is to create this table:
>
>                     School     College      Beyond
>       Husband       4          4            0
>       Wife          3          3            2
>
>
> How do I do this?
>
> I can readily do:
>     education.wife <- factor(education.wife, labels=factorlabels)
>
> But this breaks:
>     education.man <- factor(education.man,   labels=factorlabels)
>
> because none of the families have a husband who went beyond college.
>
> I get around this problem in a limited way by:
>     cautiously <- function(x, labels) {
>       factor(x, labels=factorlabels[as.numeric(levels(factor(x)))])
>     }
>     education.man <- cautiously(education.man, labels=factorlabels)
>
> Now I get:
>
>     > table(education.man)
>     School College
>          4       4
>     > table(education.wife)
>     School College  Beyond
>          3       3       2
>
> This is a pain because now the two tables are not conformable. How do
> I get to my end goal, which is the table:
>
>                     School     College      Beyond
>       Husband       4          4            0
>       Wife          3          3            2
>
> In other words, how do I force education.man to have a factor with 3
> levels - "School" "College" "Beyond" - even though there is no
> observation in "Beyond".
>
> -- 
> Ajay Shah                                                   Consultant
> ajayshah at mayin.org                      Department of Economic Affairs
> http://www.mayin.org/ajayshah           Ministry of Finance, New Delhi
>
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-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595