R-help to Exercise 21

# The following function may be used to calculate the means, variances, coefficients of dispersion.
# Pearson X2 with corresponding P-value from the three datasets.
# Copy the commands below and paste them into R:
poissontab = function(counts, x=(0:(length(counts)-1))){
  n = sum(counts)
  meanx = sum(x*counts)/n
  varx = sum((x-meanx)^2*counts)/(n-1)
  CD = varx/meanx
  df = length(counts)-2
  px = c(dpois(0:df,meanx), 1-ppois(df,meanx))
  EX = n*px
  tab = cbind(x,counts,round(EX,2), round(counts-EX,2), round((counts-EX)^2/EX,2))
  X2 = sum((counts-EX)^2/EX)
  pval = 1 - pchisq(X2,df)
  
  print(paste("Mean =",round(meanx,4)))
  print(paste("Var =",round(varx,4)))
  print(paste("CD =",round(CD,4)))
  tab = as.data.frame(tab)
  names(tab) = c("x","Obs","Exp","O-E","(O-E)^2/E")
  print(tab)
  print(paste("Pearson X2 =",round(X2,2)))
  print(paste("p-value =",round(pval,4)))
}

# In order to apply the function to the first data set in the exercise, you give the commands:
yeast = c(75,103,121,54,30,13,4)
poissontab(yeast)

moss = c(100,9,6,8,1,0,2)
poissontab(moss)

azuki = c(61,50,1)
poissontab(azuki)

#Try to understand how the function produces the output given.
# Make sure you understand the output, and use the output to answer the questions in the exercise.
# Repeat the analysis for the two other data sets in the exercise.