Your messages about masking come from attaching your data set to the R session.
In general, that is bad practice as it leads to confusing code. It is typically
better to use the ?data? argument in things like lm() to accomplish this task.
As near as I can tell, your second set of predictions is not working because
your call to lm() directly references vectors from the highdf data frame. If you
do this:
h.lm <- lm(sales ~ month, data = highdf)
news <- data.frame(month = nrow(ptr) + 1)
hcs <- predict(h.lm, news, interval = "predict")
You should see the expected results. Note that here I?m directly referring to
the variables ?sales? and ?month? and not using the bracket notation.
> On Jan 31, 2018, at 11:08 AM, WRAY NICHOLAS via R-help <r-help at
r-project.org> wrote:
>
> Hello,
>
> I am synthesising some sales data over a twelve month period, and then
trying to
> use the "predict" function, firstly to generate a thirteenth
month forecast with
> upper and lower 95% confidence limits. So far so good
>
> But what I then want to do is add the upper sales value at the 95th
confidence
> limit to the vector of thirteen months and their respective sales to create
a
> fourteenth month with a predicted sale and the 95% upper confidence limit
for
> this, and so on The idea being to create a "trumpet" of extreme
posistions
>
> But I keep getting instead of one line of predictions for the fourteenth
month,
> a whole set. What I don't understand is why it works OK with my
original
> synthetic set of twelve months, but doesn't like the set of thirteen
sales data
> points, even though as far as I can see I'm just repeating the process,
albeit
> with a different label I have tried to use different column labels in case
that
> was the problem but it doesn't seem to make any difference
>
> I am also getting these weird warning messages telling me that things are
being
> "masked":
>
> The following object is masked _by_ .GlobalEnv:
>
> sales
>
> The following object is masked from highdf (pos = 4):
>
> sales
> Etc
>
> Is it something to do with attaching the various data frames? I am a bit
at sea
> on this and would be thankful for any pointers
>
> Nick
>
> My code:
>
>
> m<-runif(1,0,1)
> m
> mres<-m*(seq(1,12))
> mres
> ssd<-rexp(1,1)
> ssd
> devs<-rep(0,length(mres))
> for(i in 1:length(mres)){devs[i]<-rnorm(1,0,ssd)}
> devs
> plot(-10,-10,xlim=c(1,24),ylim=c(0,20000))
> sales<-round((mres+devs)*1000)
>
> points(sales,pch=19)
>
> ptr<-cbind(1:length(sales),sales,sales,sales)
>
> ptr
> sdf<-data.frame(cbind(1:nrow(ptr),sales))
> sdf
>
> colnames(sdf)<-c(?monat?,?mitte?)
> sdf
> attach(sdf)
> s.lm<-lm(mitte~monat)
>
> s.lm
> abline(s.lm,lty=2)
> news<-data.frame(monat=nrow(sdf)+1)
> news
> fcs<-predict(s.lm,news,interval="predict")
> fcs
>
> points(1+nrow(ptr),fcs[,1],col="grey",pch=19)
> points(1+nrow(ptr),fcs[,2])
> points(1+nrow(ptr),fcs[,3])
> ptr<-rbind(ptr,c(1+nrow(ptr),fcs[2],fcs[1],fcs[3]))
> ptr
>
> highdf<-data.frame(ptr[,c(1,4)])
> highdf
> colnames(highdf)<-c(?month?,?sales?)
> highdf
>
> attach(highdf)
> h.lm<-lm(highdf[,2]~highdf[,1])
> h.lm
> abline(h.lm,col="gray",lty=2)
> news<-data.frame(month=nrow(ptr)+1)
> news
> hcs<-predict(h.lm,news,interval="predict")
> hcs
> [[alternative HTML version deleted]]
>
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