Add a normal distribution line in histogram

Question

I've met a weird problem that I can't figure it out totally. I'm supposed to add a normal distribution line upon a histogram.I input every step's code but after typing lines function there's no response. I don't know what's wrong.Hope anyone help me! MY code are:

grades<-mydata$Exam1
hist(grades,breaks=20,freq=T) #A correct histogram comes out.
mean(grades,na.rm=T) #there is NA in the column so I remove it when calculating mean.
[1] 75.15278

sd(grades,na.rm=T)  
[1] 16.97443 

x<-seq(0,100,0.01) 
y<-dnorm(x,mean=mean(grades,na.rm=T),sd=sd(grades,na.rm=T))
lines(x,y)#and there's no response!no line showed up!

Is anything wrong with my code? Thanks for your help!

score 21 · Accepted Answer · answered Jun 18 '13 at 07:39

21

I assume it's R code - then try this:

grades <- mydata$Exam1
hist(grades, prob=TRUE)
curve(dnorm(x, mean=mean(grades), sd=sd(grades)), add=TRUE)

Note that if you compare normal distribution to the histogram, you probably want histogram to display probabilities rather than frequencies.

answered Jun 18 '13 at 07:39

andreister

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Good point. Or he could normalize the hist$counts values returned by the function. – Carl Witthoft Jun 18 '13 at 11:25

score 4 · Answer 2 · answered Jun 18 '13 at 07:45

4

You want hist(*, freq=FALSE), not freq=TRUE.

answered Jun 18 '13 at 07:45

Hong Ooi

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? what does that have to do with his calculation of `x` and `y` ? – Carl Witthoft Jun 18 '13 at 11:24
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@carlwitthoft Nothing. The calculation of x and y is fine, the histogram is plotted on the wrong scale for the density to appear. – Hong Ooi Jun 18 '13 at 13:18

Add a normal distribution line in histogram

2 Answers2