R help - Dec 2009 - Creating Dummy Variables in R

Hi,
I am trying to create a set of dummy variables to use within a multiple linear
regression and am unable to find the codes within the manuals.

For example i have:
Price     Weight     Clarity
                             IF      VVS1    VVS2
500        8             1         0          0
1000      5.2          0         0          1
864        3              0        1          0
340        2.6          0         0          1
90          0.5          1         0          0 
450        2.3          0         1          0

Where price is dependent upon weight (single value in each observation) and
clarity (split into three levels, IF, VVS1, VVS2).
I am having trouble telling the program that clarity is a set of 3 dummy
variables and keep getting error messages, what is the correct way?

Any helps is greatly appreciated.
Matthew

Hi

r-help-bounces at r-project.org napsal dne 16.12.2009 15:58:56:
> Hi,
> I am trying to create a set of dummy variables to use within a multiple 
linear
> regression and am unable to find the codes within the manuals.
> 
> For example i have:
> Price     Weight     Clarity
>                              IF      VVS1    VVS2
> 500        8             1         0          0
> 1000      5.2          0         0          1
> 864        3              0        1          0
> 340        2.6          0         0          1
> 90          0.5          1         0          0 
> 450        2.3          0         1          0
> 
> Where price is dependent upon weight (single value in each observation) 
and 
> clarity (split into three levels, IF, VVS1, VVS2).
> I am having trouble telling the program that clarity is a set of 3 dummy 
> variables and keep getting error messages, what is the correct way?
Well, try to bribe it. Or ask what please it to break its resistance.

Seriously. What is a structure of your data in R.

?str

what commands did you use for regression

I suppose

lm(Price~Weight+IF+VVS1+VVS2, data=your.data)

shall not complain if your.data is a data frame.

Regards
Petr


> 
> Any helps is greatly appreciated.
> Matthew
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide 
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

On 12/16/2009 03:58 PM, whitaker m. (mw1006) wrote:
> Hi,
> I am trying to create a set of dummy variables to use within a multiple
linear regression and am unable to find the codes within the manuals.
> 
> For example i have:
> Price     Weight     Clarity
>                              IF      VVS1    VVS2
> 500        8             1         0          0
> 1000      5.2          0         0          1
> 864        3              0        1          0
> 340        2.6          0         0          1
> 90          0.5          1         0          0 
> 450        2.3          0         1          0
> 
> Where price is dependent upon weight (single value in each observation) and
clarity (split into three levels, IF, VVS1, VVS2).
> I am having trouble telling the program that clarity is a set of 3 dummy
variables and keep getting error messages, what is the correct way?
> 
Without an example of your code, it's a bit difficult. But it might be
easier to use one variable "clarity" with three possible values (IF,
VVS1, VVS2), defined as a factor.
lm(Price ~ Weight + Clarity) should then do the trick (unless you
explicitly want to use a different dummy coding than the default)

Stephan

On Wed, 16 Dec 2009, whitaker m. (mw1006) wrote:
> Hi,
> I am trying to create a set of dummy variables to use within a multiple
linear regression and am unable to find the codes within the manuals.
>
> For example i have:
> Price     Weight     Clarity
>                             IF      VVS1    VVS2
> 500        8             1         0          0
> 1000      5.2          0         0          1
> 864        3              0        1          0
> 340        2.6          0         0          1
> 90          0.5          1         0          0
> 450        2.3          0         1          0
>
> Where price is dependent upon weight (single value in each observation) and
clarity (split into three levels, IF, VVS1, VVS2).
> I am having trouble telling the program that clarity is a set of 3 dummy
variables and keep getting error messages, what is the correct way?
You should code the categorical variable "Clarity" as a
"factor" so that R
knows that this is a categorical variable and can deal with it 
appropriately in subsequent computations such as summary() or lm().

Thus, I would recommend to store your data as

dat <- data.frame(
   Price = c(500, 1000, 864, 340, 90, 450),
   Weight = c(8, 5.2, 3, 2.6, 0.5, 2.3),
   Clarity = c("IF", "VVS1", "VVS2")[c(1, 3, 2, 3,
1, 2)])

which yields, e.g.,

R> summary(dat)
      Price            Weight      Clarity
  Min.   :  90.0   Min.   :0.500   IF  :2
  1st Qu.: 367.5   1st Qu.:2.375   VVS1:2
  Median : 475.0   Median :2.800   VVS2:2
  Mean   : 540.7   Mean   :3.600
  3rd Qu.: 773.0   3rd Qu.:4.650
  Max.   :1000.0   Max.   :8.000

and then you can also do

R> lm(Price ~ Weight + Clarity, data = dat)

Call:
lm(formula = Price ~ Weight + Clarity, data = dat)

Coefficients:
(Intercept)       Weight  ClarityVVS1  ClarityVVS2
      -45.05        80.01       490.02       403.00

or if you wish to choose a different coding

R> lm(Price ~ 0 + Weight + Clarity, data = dat)

Call:
lm(formula = Price ~ 0 + Weight + Clarity, data = dat)

Coefficients:
      Weight    ClarityIF  ClarityVVS1  ClarityVVS2
       80.01       -45.05       444.97       357.95


Some further reading of introductory material on linear regression in R 
would be useful. Also look at ?lm, ?factor, ?model.matrix, ?contrasts etc.

hth,
Z
> Any helps is greatly appreciated.
> Matthew
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
>

Is your variable Clarity a categorical with 4 levels? Thus, the need for
k-1 (3) dummies? Your error may be the result of creating k instead of
k-1 dummies, but can't be sure from the example.

In R, you don't have to (unless you really want to) explicitly create
separate variables. You can use the internal contrast functions. 

See

?contr.treatment

Which is dummy coding by default. You can specify which group is the
reference group. 

Alternatively, if you prefer effects coding, you can see
?contr.sum 

There are others as well. 

Tom Fletcher



-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of whitaker m. (mw1006)
Sent: Wednesday, December 16, 2009 8:59 AM
To: r-help at r-project.org
Subject: [R] Creating Dummy Variables in R

Hi,
I am trying to create a set of dummy variables to use within a multiple
linear regression and am unable to find the codes within the manuals.

For example i have:
Price     Weight     Clarity
                             IF      VVS1    VVS2
500        8             1         0          0
1000      5.2          0         0          1
864        3              0        1          0
340        2.6          0         0          1
90          0.5          1         0          0 
450        2.3          0         1          0

Where price is dependent upon weight (single value in each observation)
and clarity (split into three levels, IF, VVS1, VVS2).
I am having trouble telling the program that clarity is a set of 3 dummy
variables and keep getting error messages, what is the correct way?

Any helps is greatly appreciated.
Matthew

______________________________________________
R-help at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.