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#
#  Script: 15 February 2011 (20110215.R)
#
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# Today:
#   ** Creating functions (in R)
#   ** Categorical vs Categorical 
#      . Two-Way


# Next time:
#   ** Statistical effect
#      . Start of level of effect
#   ** Observed distribution vs. Expected distribution
#      . One-Way



# Writing functions 

# Step 1: What should the function give to me?
# Step 2: What information do I need to provide the function?
# Step 3: How do I get from 1 to 2?
# Step 4: Algorithm
# Step 5: Program
# Step 6: Test, Test, Test, Test, Test, Test!



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#
#   Theme: The Robust measure (R)
#

# Example 1: Arithmetic Mean 
mean.a <- function(var) {
  n <- length(var)
  m <- sum(var)/n
  return(m)
}


# Example 2: Harmonic Mean
mean.h <- function(var) {
  n <- length(var)
  m <- n/sum(1/var)
  return(m)
}


# Example 3: Geometric Mean
mean.g <- function(var) {
  n <- length(var)
  m <- exp( 1/n * sum( log(var) ) )
  return(m)
}


# Example 4: Median
med <- function(var) {
  n <- length(var)
  var <- sort(var)
  if( n/2 == floor(n/2) ) {
    m <- 0.5*( var[n/2] + var[n/2+1])
  } else {
    m <- var[ (n+1)/2 ]
  }
  return(m)
}