Hi, I want to plot ROC curve for my detection algorithm which detects features in different images at two different thresholds. 6 different images used and obtained tp, fp and fn. No tn in my case. in first threshold run i obtained 6 values of tp,fp and fn. In second threshold run agian i got 6 more. i had calculated tpr and fpr. How to plot ROC in this case. Is this possible to plot ROC curve in this case? If s, please guide me to plot that. thank you. http://r.789695.n4.nabble.com/file/n4640474/in1.png -- View this message in context: http://r.789695.n4.nabble.com/no-true-negative-data-need-roc-curve-tp4640474.html Sent from the R help mailing list archive at Nabble.com.
To clarify: Is TN = 0 or do you not know TN (N)? On 16.08.2012, at 11:51, vjyns wrote:> Hi, > > I want to plot ROC curve for my detection algorithm which detects > features in different images at two different thresholds. > > 6 different images used and obtained tp, fp and fn. No tn in my case. > > in first threshold run i obtained 6 values of tp,fp and fn. In second > threshold run agian i got 6 more. > > i had calculated tpr and fpr. How to plot ROC in this case. > > Is this possible to plot ROC curve in this case? If s, please guide me to > plot that. > > thank you. > > http://r.789695.n4.nabble.com/file/n4640474/in1.png > > > > -- > View this message in context: http://r.789695.n4.nabble.com/no-true-negative-data-need-roc-curve-tp4640474.html > Sent from the R help mailing list archive at Nabble.com. > > ______________________________________________ > 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.
TN=0 in all cases, i had only tp, fp and fn for 6 images (two sets). suggest me how plot the roc curve. -- View this message in context: http://r.789695.n4.nabble.com/no-true-negative-data-need-roc-curve-tp4640474p4640558.html Sent from the R help mailing list archive at Nabble.com.
Okay, first of a point in ROC space is spanned by true positive rate and false positive rate. Now you need to decide if you want to plot a curve for each image or maybe a curve for the complete data (add all together). In your case you only have 2 thresholds, so that makes the "curves" a little awkward because you only have 4 points total, but whatever. Lets say you want a curve for image1: tp1<-1066 fp1<-70 fn1<-116 tn1<-0 tpr1<-tp1/(tp1+fn1) fpr1<-fp1/(fp1+tn1) tp2<-446 fp2<-41 fn2<-55 tn2<-0 tpr2<-tp2/(tp2+fn2) fpr2<-fp2/(fp2+tn2) plot(0,0,type="n",ylim=c(0,1),xlim=c(0,1)) points(c(0,fpr1,fpr2,1),c(0,tpr1,tpr2,1)) lines(c(0,fpr1,fpr2,1),c(0,tpr1,tpr2,1)) Now i have to say though, after taking a look at that data you provided: THE DATA IS WRONG If you add up all values (tp+tn+fp+fn), the number in threshold 2 is lower than in threshold 1. It needs to be the same, so what the hell have you DONE? ^^ greetings Jessi On 16.08.2012, at 17:49, Jessica Streicher wrote:> To clarify: > > Is TN = 0 or do you not know TN (N)? > > On 16.08.2012, at 11:51, vjyns wrote: > >> Hi, >> >> I want to plot ROC curve for my detection algorithm which detects >> features in different images at two different thresholds. >> >> 6 different images used and obtained tp, fp and fn. No tn in my case. >> >> in first threshold run i obtained 6 values of tp,fp and fn. In second >> threshold run agian i got 6 more. >> >> i had calculated tpr and fpr. How to plot ROC in this case. >> >> Is this possible to plot ROC curve in this case? If s, please guide me to >> plot that. >> >> thank you. >> >> http://r.789695.n4.nabble.com/file/n4640474/in1.png >> >> >> >> -- >> View this message in context: http://r.789695.n4.nabble.com/no-true-negative-data-need-roc-curve-tp4640474.html >> Sent from the R help mailing list archive at Nabble.com. >> >> ______________________________________________ >> 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. > > ______________________________________________ > 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.
Hi,
thanks for the quick response, but as i said in my case due to two
different threshold the detected features will differ. Moreover, there is
some standard /refined/ formula in calculating the tpr and fpr. herewith i
had attached the refined formula from a standard international journal
http://r.789695.n4.nabble.com/file/n4640577/tpr_and_fpr.jpg
when i used the above mentioned formula (fpr=fp/fp+tp) then i can able to
see my point are distributed on the extreme left corner. Like this it is
possible to put all the 6 images. Will you please suggest me now how to
obtain the plot for different images of two threshold.
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Above mentioned formula is wrong - maybe a typo http://en.wikipedia.org/wiki/Receiver_operating_characteristic The false positive rate is the rate of false positives, meaning how many of the total negatives (all in reality negatives(N), that is, all negatives falsely classified as positives(fp) and all negatives correctly classified as negatives(tn)) have been falsely classified as positive. Also the authors obviously had (N+P=number of features), and therefore at least could have computed this properly. For example: N+P=100 P=TP+FN N=FP+TN -> do the math with what you got On 17.08.2012, at 11:13, vjyns wrote:> Hi, > > thanks for the quick response, but as i said in my case due to two > different threshold the detected features will differ. Moreover, there is > some standard /refined/ formula in calculating the tpr and fpr. herewith i > had attached the refined formula from a standard international journal > http://r.789695.n4.nabble.com/file/n4640577/tpr_and_fpr.jpg > > when i used the above mentioned formula (fpr=fp/fp+tp) then i can able to > see my point are distributed on the extreme left corner. Like this it is > possible to put all the 6 images. Will you please suggest me now how to > obtain the plot for different images of two threshold. > > > > -- > View this message in context: http://r.789695.n4.nabble.com/no-true-negative-data-need-roc-curve-tp4640474p4640577.html > Sent from the R help mailing list archive at Nabble.com. > > ______________________________________________ > 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.