# brief exercise 2
ii<-read.csv('nhanes.csv')
ii$pounds<-ii$bmxwt*2.20462
head(ii$pounds) # just a check
head(ii$bmxwt) # just a check
ii$inches<-ii$bmxht*0.0328084
head(ii$inches) # just a check
head(ii$bmxht) # just a check
names(ii) # just a check

# brief exercise 3
mean(ii$lbdhdd,na.rm=TRUE)
sd(ii$lbdhdd,na.rm=TRUE)
quantile(ii$lbdhdd,c(0.2,0.4,0.6,0.8),na.rm=TRUE)
quantile(ii$lbdhdd,seq(0.2,0.8,0.2),na.rm=TRUE) # equivalent to above
hist(ii$lbdhdd)

# brief exercise 4
table(ii$ldlcat,useNA='always') # to see current levels and current order
ii$ldlcat<-factor(ii$ldlcat,c('Optimal','Near optimal','Borderline high',
                              'High','Very high'))
table(ii$ldlcat,useNA='always') # just a check
table(ii$ldlcat)
prop.table(table(ii$ldlcat))*100

# brief exercise 5
mean(ii$lbdldl,na.rm=TRUE)
t.test(ii$lbdldl,mu=100)