### Section 6.3 A Model for the Normal Distribution

#This section describes what produces a normal distribution and

#a heuristic showing how it is related to the binomial distribution.

### Section 6.4 Applications of the Normal Distribution

#This section describes how to do what we did to get Table 6.1,

#but with a different set of data back from Section 4.2.

mean.bw<-109.9

sd.bw<-13.593

(sdeviate.bw<-(151-mean.bw)/sd.bw)

pnorm(sdeviate.bw) #This value is more precise

pnorm(3.02) #But the book uses this rounded value so we will too here.

#We can do some of the calculations the book does on pp. 103-104.

#Only a very few individuals are higher than 151 oz.

1-pnorm(3.02)

#If we want to look both directions we can double this number since the distribution is symmetrical.

2*(1-pnorm(3.02))

#By default, lower.tail=TRUE in pnorm.

pnorm(3.02,

lower.tail=FALSE)

#It is the same as our earlier value of

1-pnorm(3.02)

#Here is the default setting for comparison.

pnorm(3.02,

lower.tail=TRUE)

#This simply tells use which direction we want to look at,

#the upper or lower tail of the distribution from our value of standard deviate.

### Section 6.5 Fitting a Normal Distribution to Observed Data

#These data are again from Section 4.2.

classmark<-seq(from=59.5, to=171.5, by=8)

frequencies<-c(2,6,39,385,888,1729,2240,2007,1233,641,201,74,14,5,1)

samplesize<-sum(frequencies) #This confirms that we entered the data correctly, and gets our sample size.

#Multiply classmark and frequencies to get the sums for each class.

classsums<-classmark*frequencies

#To look at all this stuff together, combine it into a dataset.

birthweights<-data.frame(cbind(classmark, frequencies, classsums))

#Add on a row of the next class up which contains 0 individuals.

(birthweights<-rbind(birthweights, c(179.5, 0, 0)))

#On page 104, equation 6.2 is like equation 6.1 above but with sample size (n) and i (class intervals).

normal.manual.applied<-function(mean, sd, n, i){

curve(((1/(sd*sqrt(2*pi)))*exp((-((x-mean)/sd)^2)/2))*n*i,

-4, 4, #go from -4 to +4 standard deviations.

add = FALSE,

ylab="freq",

xlab="Y",

type = "l")

}

normal.manual.applied(mean=0, sd=1, n=1000, i=0.5)

#This gives the curve that Table 6.1 also has (same class intervals of 0.5, 1000 samples, and mean=0 with sd=1).

#Let's do this with the birthweights data.

birthweights.mean<-109.8996

birthweights.sd<-13.5942

#Like above we need the lower boundaries of the class marks.

birthweights$boundaries<-birthweights$classmark-4

#Get the expected frequencies for the class boundaries with the mean and sd of our dataset.

birthweights$pnorm.results<-pnorm(birthweights$boundaries,

mean=birthweights.mean,

sd=birthweights.sd)

#Then, take the difference of the first row minus the next row.

#The last row will not have anything, which is why we needed to add the lower boundary of

#the next class mark, which has a frequency of zero. Thus, this calculation generates a vector of length 14.

#We need 15, so we just add a zero on for the last difference as they do in Table 6.2

birthweights$expected.freqs<-c(abs(diff(birthweights$pnorm.results)),0) #add a zero on for the last difference

#Multipy the frequencies by the sample size to get the expected frequencies for a sample of this size.

#Round as in the table.

birthweights$expected.freqs.values<-round(birthweights$expected.freqs*samplesize, 1)

#We can even add the plus and minus signs using ifelse and sign() to see in which direction the differences are.

birthweights$departure.signs<-ifelse(sign(birthweights$frequencies-birthweights$expected.freqs.values)==-1,

"-", #if -1, then write "-"

"+") #else if not -1, write "+"

#View the table to confirm it has the same data as Table 6.2.

birthweights

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