R gives me the error "contrasts can be applied only to factors with 2 or more levels" running an mlogit model, but all my factors have 2 levels

Question

I have a set of results from a choice experiment that I am trying to analyse using the mlogit function in R. My data set is shown below, where each STR value represents the three choices in a single choice experiment, 'CHOICE' is a binary variable indicating which the participant selected, and columns C-G detail the attributes of each choice (D-G being binary variables - 1=yes, 0=no).

image of data set

I am trying to run the model as:

m <- mlogit(CHOICE ~ PRICE + GreenStatement + Certification +
            MediumBrand + HighBrand,
            SP1, choice = "CHOICE", shape = "long", alt.levels = "STR")

but get the error "contrasts can be applied only to factors with 2 or more levels". I have tried running 'as.factor' on all the variables, so they are different factors, and asking R to display a data summary tells me that they all have 2 or more levels. But I still get this error. Would anyone please be able to help me identify (and fix!) the source of this error?

Thanks.

Are you sure that everything you think is a factor is? try putting as.factor into the mlogit function, as in mlogit(CHOICE ~ PRICE + as.factor(GreenStatement) ... — Jeremy Miles, Mar 13 '15 at 23:06
What do `is.factor(SP1$CHOICE)` and `unique(SP1$CHOICE)` and `levels(SP1$CHOICE)` return? — shadowtalker, Mar 14 '15 at 10:59
Do that for all of them. Especially `MediumBrand` since from the screen cap it looks like a column of zeroes — shadowtalker, Mar 14 '15 at 11:01
They all give me the same output: is.factor(VARIABLE) [1] TRUE ; unique(VARIABLE) [1] 0 1 Levels: 0 1 ; levels(VARIABLE) [1] "0" "1" — elephants_neve_forget, Mar 15 '15 at 10:11
The help on `mlogit` doesn't list an `alt.levels` argument -- what does it do? — Glen_b, Nov 17 '15 at 06:14

score 10 · Answer 1 · edited May 23 '17 at 12:39

10

See Answer Here - https://stackoverflow.com/questions/18171246/error-in-contrasts-when-defining-a-linear-model-in-r

There are factors that you are using that either have only 1 distinct value or are NA

edited May 23 '17 at 12:39

Community

1

answered Aug 05 '15 at 10:14

myloginid

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And you can check by using, say, summary() on all of your factor variables. – Jordan Collins Feb 22 '16 at 16:35
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Here's a way to check this if `dt` is a `data.table`: `char.factor.cols – Max Ghenis Mar 24 '17 at 22:01
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for a plain data frame, `df`, you can use: `which(sapply(df, function(x) (is.character(x) | is.factor(x)) & length(unique(x))<2))` – Roobie Nuby May 14 '17 at 23:12

R gives me the error "contrasts can be applied only to factors with 2 or more levels" running an mlogit model, but all my factors have 2 levels

1 Answers1