[R] Problem with predict?

[Petr PIKAL]

Hi

> 
> Hello,
> 
> I am trying to fit a model to some "death over time" data that does not 
> fit the criteria for the usual LD50 type models (the counts are too 
> large). I am using a simple linear model in an attempt to plot a nice 
line
> on a scatter plot and calculate some LD values to use in designing an 
> experiment. Here is the basic idea of what I'm doing:
> 
> 
> head(mort)
> 
> Time    Density
> 0        2233333333
> 0        2100000000
> 0        1933333333
> 5        1900000000
> 5        1433333333
> 5         900000000
> 
> 
> plot(Density~Time)
> 
> This plots something that looks a lot like a decay rate
> 
> mod<-lm(log(Density)~Time)
> 
> xv<-seq(0,60,0.1)
> yv<-exp(predict(mod,list(time=xv)))

>From help page

Usage
## S3 method for class 'lm'
predict(object, newdata, se.fit = FALSE, scale = NULL, df = Inf,
        interval = c("none", "confidence", "prediction"),
        level = 0.95, type = c("response", "terms"),
        terms = NULL, na.action = na.pass,
        pred.var = res.var/weights, weights = 1, ...)

Arguments
object Object of class inheriting from "lm"
 
newdata An optional data frame in which to look for variables with which 
to predict. If omitted, the fitted values are used.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 

yv<-exp(predict(mod, data.frame(time=xv)))

shall work.

Regards
Petr

> lines(xv,yv)
> 
> Everything seems to work fine until I try to plot the lines, but then I 
> get the error message:
> 
> Error in xy.coords(x, y) : 'x' and 'y' lengths differ
> 
> Checking the lengths of x and y confirms that somehow the object yv is 
not
> using xv to predict the data, and is only predicting as many data points 

> as there are rows in the data frame.
> 
> Any ideas on why this might be would be very much appreciated.
> 
> Thank you,
> 
> Julie
> 
> 
> 
> 
> 
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> 
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