How to retrieve overall accuracy value from confusionMatrix in R?
Given a confusion matrix cm
, the overall accuracy is obtained by overall.accuracy <- cm$overall['Accuracy']
It's the first time I see the caret
package, so how did I know this?
Since you didn't provide an example, I searched for an example code for caret confusion matrices. Here it is (I only added assignment in the last statement):
###################
## 3 class example
confusionMatrix(iris$Species, sample(iris$Species))
newPrior <- c(.05, .8, .15)
names(newPrior) <- levels(iris$Species)
cm <- confusionMatrix(iris$Species, sample(iris$Species))
Now, let's take a look what's in the confusion matrix:
> str(cm)
List of 5
$ positive: NULL
$ table : 'table' int [1:3, 1:3] 13 18 19 20 13 17 17 19 14
..- attr(*, "dimnames")=List of 2
.. ..$ Prediction: chr [1:3] "setosa" "versicolor" "virginica"
.. ..$ Reference : chr [1:3] "setosa" "versicolor" "virginica"
$ overall : Named num [1:7] 0.267 -0.1 0.198 0.345 0.333 ...
..- attr(*, "names")= chr [1:7] "Accuracy" "Kappa" "AccuracyLower" "AccuracyUpper" ...
$ byClass : num [1:3, 1:8] 0.26 0.26 0.28 0.63 0.63 0.64 0.26 0.26 0.28 0.63 ...
..- attr(*, "dimnames")=List of 2
.. ..$ : chr [1:3] "Class: setosa" "Class: versicolor" "Class: virginica"
.. ..$ : chr [1:8] "Sensitivity" "Specificity" "Pos Pred Value" "Neg Pred Value" ...
$ dots : list()
- attr(*, "class")= chr "confusionMatrix"
As you may see, the cm
object is a list. We see various "byClass" and "overall" statistics. The overall part is obtained by:
overall <- cm$overall
Which gives us a vector of numbers with string indices:
> overall
Accuracy Kappa AccuracyLower AccuracyUpper AccuracyNull AccuracyPValue McnemarPValue
0.2666667 -0.1000000 0.1978421 0.3449492 0.3333333 0.9674672 0.9547790
Now, extracting the relevant value is as simple as:
> overall.accuracy <- overall['Accuracy']
Summary: str
is your friend. Another useful function is attributes
-- it returns all the attributes of a given object.
user697911
Updated on July 13, 2022Comments
-
user697911 almost 2 years
In R caret library, if I got a confusion matrix like this below, if there a way to retrieve the overall accuracy 0.992? I can't get this single value out, since I need to store this value and use it for later processing. Is this possible at all?
Prediction A B C D E A 1114 2 0 0 0 B 9 745 5 0 0 C 0 6 674 4 0 D 0 0 3 640 0 E 0 0 2 1 718
Overall Statistics
Accuracy : 0.992 95% CI : (0.989, 0.994) No Information Rate : 0.286 P-Value [Acc > NIR] : <2e-16 Kappa : 0.99
Mcnemar's Test P-Value : NA
Statistics by Class:
Class: A Class: B Class: C Class: D Class: E Sensitivity 0.992 0.989 0.985 0.992 1.000 Specificity 0.999 0.996 0.997 0.999 0.999 Pos Pred Value 0.998 0.982 0.985 0.995 0.996 Neg Pred Value 0.997 0.997 0.997 0.998 1.000 Prevalence 0.286 0.192 0.174 0.164 0.183 Detection Rate 0.284 0.190 0.172 0.163 0.183 Detection Prevalence 0.284 0.193 0.174 0.164 0.184 Balanced Accuracy 0.996 0.992 0.991 0.996 1.000
-
Emixam23 about 7 yearsNice answer but accuracy return a string and a value, how can I just access the value? I mean the double
-
Saahil almost 5 years@Emixam23 you might have figured out by now the solution for your question, but for someone who is looking for answer, just add another square brackets in front and back of the brackets mentioned above, like this
overall.accuracy <- overall[['Accuracy']]