[R] Probit Analysis
apjaworski@mmm.com
apjaworski at mmm.com
Wed Jan 1 01:38:02 CET 2003
Jacquelin,
Here is an answer to your question wrapped in much longer question of my
own.
Cheers,
Andy

Extracting components from complex objects is, in my opinion, not easy to
do unless I am missing something very obvious. Take the glm.dose object as
an example. Following the example on the dose.p help page I ran
zz < dose.p(budworm.lg0, cf = c(1,3), p = 1:3/4)
Typing zz applies print method to the zz object and shows
Dose SE
p = 0.25: 2.231264 0.2498235
p = 0.50: 3.263587 0.2297039
p = 0.75: 4.295910 0.2746190
If I now want to extract some of the numbers from zz, I have to go through
some hoops. What I usually do first is
str(zz)
This gives me
Class 'glm.dose' atomic [1:3] 2.23 3.26 4.30
.. attr(*, "SE")= num [1:3, 1] 0.250 0.230 0.275
.. .. attr(*, "dimnames")=List of 2
.. .. ..$ : chr [1:3] "p = 0.25:" "p = 0.50:" "p = 0.75:"
.. .. ..$ : NULL
.. attr(*, "p")= num [1:3] 0.25 0.5 0.75
The format of this output is not really described on the str help page, so
I am guessing that what this means is that the "basic" object is a vector
of length 3 with a couple of attributes called SE and p, the SE attribute
having additional nested attribute called dimnames. We can onfirm that by
attributes(zz)
giving
$names
[1] "p = 0.25:" "p = 0.50:" "p = 0.75:"
$SE
[,1]
p = 0.25: 0.2498235
p = 0.50: 0.2297039
p = 0.75: 0.2746190
$p
[1] 0.25 0.50 0.75
$class
[1] "glm.dose"
So, If I want to extract the actual dose value from zz I just need
zz[1]
geting
p = 0.25:
2.231264
but if I want to get the first standard error I have to do
attributes(zz)$SE[1]
getting
p = 0.25:
0.2498235
Is this a correct general method of extracting components from complex
objects? Is there a simpler way?
Thanks in advance and sorry for this longish post, but this is something I
have been having problems with for a long time and it seems that every time
I run into this "extraction problem" I have to reinvent the wheel.
Andy
__________________________________
Andy Jaworski
Engineering Systems Technology Center
3M Center, 518101
St. Paul, MN 551441000

Email: apjaworski at mmm.com
Tel: (651) 7336092
Fax: (651) 7363122
+>
  "Ngayee J Law" 
  <Ngayee.Law at celeradiagn
  ostics.com> 
  
  12/31/2002 17:18 
  
+>
>
 
 To: apjaworski at mmm.com 
 cc: rhelp at stat.math.ethz.ch 
 Subject: Re: [R] Probit Analysis 
>
Hi Andy,
Thanks! glm and dose.p can do exaclty what I want to do. Just a quick
question,
how can I extract the numbers in the glm.dose object created by the dose.p?
My
goal is to calculate a 95% CI for the 'LD50'.
 Jacqueline
apjaworski at mm
m.com To: "Ngayee J Law"
<Ngayee.Law at celeradiagnostics.com>
cc: rhelp at stat.math.ethz.ch
12/31/2002 Subject: Re: [R] Probit
Analysis
02:47 PM
Jacqueline,
This should be a simple application of GLM. Check the help pages for glm,
family and dose.p in library MASS. Look at the example at the bottom of
the dose.p help page  it does almost exactly what you need.
Hope this helps,
Andy
__________________________________
Andy Jaworski
Engineering Systems Technology Center
3M Center, 518101
St. Paul, MN 551441000

Email: apjaworski at mmm.com
Tel: (651) 7336092
Fax: (651) 7363122
+>
  "Ngayee J Law" 
  <Ngayee.Law at celeradiagn
  ostics.com> 
  Sent by: 
  rhelpadmin at stat.math.
  ethz.ch 
  
  
  12/31/2002 16:17 
  
+>
>



 To: rhelp at stat.math.ethz.ch

 cc:

 Subject: [R] Probit Analysis

>

Hello all,
I have a very simple set of data and I would like to analyze them with
probit analysis.
The data are:
X Event Trial
100 8 8
75 8 8
50 6 8
25 4 8
10 2 8
0 0 8
I want to estimate the value of X that will give a 95% hit rate
(Event/Trial) and the corresponding
95% CI. Anyone can offer some help? Thanks!!
 Jacqueline
______________________________________________
Rhelp at stat.math.ethz.ch mailing list
http://www.stat.math.ethz.ch/mailman/listinfo/rhelp
More information about the Rhelp
mailing list