R: numeric 'envir' arg not of length one in predict()

Question

I'm trying to predict a value in R using the predict() function, by passing along variables into the model.

I am getting the following error:

Error in eval(predvars, data, env) :    numeric 'envir' arg not of length one 

Here is my data frame, name df:

df <- read.table(text = '      Quarter Coupon      Total 1   "Dec 06"  25027.072  132450574 2   "Dec 07"  76386.820  194154767 3   "Dec 08"  79622.147  221571135 4   "Dec 09"  74114.416  205880072 5   "Dec 10"  70993.058  188666980 6   "Jun 06"  12048.162  139137919 7   "Jun 07"  46889.369  165276325 8   "Jun 08"  84732.537  207074374 9   "Jun 09"  83240.084  221945162 10  "Jun 10"  81970.143  236954249 11  "Mar 06"   3451.248  116811392 12  "Mar 07"  34201.197  155190418 13  "Mar 08"  73232.900  212492488 14  "Mar 09"  70644.948  203663201 15  "Mar 10"  72314.945  203427892 16  "Mar 11"  88708.663  214061240 17  "Sep 06"  15027.252  121285335 18  "Sep 07"  60228.793  195428991 19  "Sep 08"  85507.062  257651399 20  "Sep 09"  77763.365  215048147 21  "Sep 10"  62259.691  168862119', header=TRUE)   str(df) 'data.frame':   21 obs. of  3 variables:  $ Quarter   : Factor w/ 24 levels "Dec 06","Dec 07",..: 1 2 3 4 5 7 8 9 10 11 ...  $ Coupon: num  25027 76387 79622 74114 70993 ...  $ Total: num  132450574 194154767 221571135 205880072 188666980 ... 

Code:

model <- lm(df$Total ~ df$Coupon)  > model  Call: lm(formula = df$Total ~ df$Coupon)  Coefficients: (Intercept)    df$Coupon     107286259         1349  

Now, when I run predict, I get the error I showed above.

> predict(model, df$Total, interval="confidence") Error in eval(predvars, data, env) :    numeric 'envir' arg not of length one 

Any idea where I am going wrong?

Thanks

Answer 1

There are several problems here:

1. The newdata argument of predict() needs a predictor variable. You should thus pass it values for Coupon, instead of Total, which is the response variable in your model.

2. The predictor variable needs to be passed in as a named column in a data frame, so that predict() knows what the numbers its been handed represent. (The need for this becomes clear when you consider more complicated models, having more than one predictor variable).

3. For this to work, your original call should pass df in through the data argument, rather than using it directly in your formula. (This way, the name of the column in newdata will be able to match the name on the RHS of the formula).

With those changes incorporated, this will work:

model <- lm(Total ~ Coupon, data=df) new <- data.frame(Coupon = df$Coupon) predict(model, newdata = new, interval="confidence")