# [R] newdata for predict.lm() ??

Achim Zeileis Ach|m@Ze||e|@ @end|ng |rom u|bk@@c@@t
Wed Nov 4 11:05:31 CET 2020

```On Wed, 4 Nov 2020, peter dalgaard wrote:

> Don't use \$ notation in lm() formulas. Use lm(w ~ h, data=DAT).

...or in any other formula for that matter!

Let me expand a bit on Peter's comment because this is really a pet peeve
of mine:

The idea is that the formula "w ~ h" described the relationships between
the variables involved, independent of the data set this should be applied
to. In contrast "DAT\$w ~ DAT\$h" hard-wires the data into the formula and
prevents it from applying the formula to another data set.

Hope that helps,
Achim

>> On 4 Nov 2020, at 10:50 , Boris Steipe <boris.steipe using utoronto.ca> wrote:
>>
>> Can't get data from a data frame into predict() without a detour that seems quite unnecessary ...
>>
>> Reprex:
>>
>> # Data frame with simulated data in columns "h" (independent) and "w" (dependent)
>> DAT <- structure(list(h = c(2.174, 2.092, 2.059, 1.952, 2.216, 2.118,
>>                            1.755, 2.060, 2.136, 2.126, 1.792, 1.574,
>>                            2.117, 1.741, 2.295, 1.526, 1.666, 1.581,
>>                            1.522, 1.995),
>>                      w = c(90.552, 89.518, 84.124, 94.685, 94.710, 82.429,
>>                            87.176, 90.318, 76.873, 84.183, 57.890, 62.005,
>>                            84.258, 78.317,101.304, 64.982, 71.237, 77.124,
>>                            65.010, 81.413)),
>>                 row.names = c( "1",  "2",  "3",  "4",  "5",  "6",  "7",
>>                                "8",  "9", "10", "11", "12", "13", "14",
>>                               "15", "16", "17", "18", "19", "20"),
>>                 class = "data.frame")
>>
>>
>> myFit <- lm(DAT\$w ~ DAT\$h)
>> coef(myFit)
>>
>> # (Intercept)       DAT\$h
>> #   11.76475    35.92002
>>
>>
>> # Create 50 x-values with seq() to plot confidence intervals
>> myNew <- data.frame(seq(min(DAT\$h), max(DAT\$h), length.out = 50))
>>
>> pc <- predict(myFit, newdata = myNew, interval = "confidence")
>>
>> # Warning message:
>> # 'newdata' had 50 rows but variables found have 20 rows
>>
>> # Problem: predict() was not able to take the single column in myNew
>> # as the independent variable.
>>
>> # Ugly workaround: but with that everything works as expected.
>> xx <- DAT\$h
>> yy <- DAT\$w
>> myFit <- lm(yy ~ xx)
>> coef(myFit)
>>
>> myNew <- data.frame(seq(min(DAT\$h), max(DAT\$h), length.out = 50))
>> colnames(myNew) <- "xx"  # This fixes it!
>>
>> pc <- predict(myFit, newdata = myNew, interval = "confidence")
>> str(pc)
>>
>> # So: specifying the column in newdata to have same name as the coefficient
>> # name should work, right?
>> # Back to the original ...
>>
>> myFit <- lm(DAT\$w ~ DAT\$h)
>> colnames(myNew) <- "`DAT\$h`"
>> # ... same error
>>
>> colnames(myNew) <- "h"
>> # ... same error again.
>>
>> Bottom line: how can I properly specify newdata? The documentation is opaque. It seems the algorithm is trying to EXACTLY match the text of the RHS of the formula, which is unlikely to result in a useful column name, unless I assign to an intermediate variable. There must be a better way ...
>>
>>
>>
>> Thanks!
>> Boris
>>
>> ______________________________________________
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>> and provide commented, minimal, self-contained, reproducible code.
>
> --
> Peter Dalgaard, Professor,
> Center for Statistics, Copenhagen Business School
> Solbjerg Plads 3, 2000 Frederiksberg, Denmark
> Phone: (+45)38153501
> Office: A 4.23
> Email: pd.mes using cbs.dk  Priv: PDalgd using gmail.com
>
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