Explore Confidence Interval, Two Pop., Diff means, σ's known


The script below provides a way to
  1. Create two populations of values with specified means and standard deviations.
  2. Find the mean and standard deviation of each population.
  3. Specify the size of a sample to be taken from each population.
  4. Specify the confidence level to use.
  5. Specify the number of times to take such samples.
  6. Perform the sampling and, for each sample, generate a confidence interval for the difference of the population means.
  7. Keep track of the number of times that the generated confidence interval actually contains the true mean difference.
  8. Report that count.
  9. Report the standard deviation of the collection of the differences of the two sample means.
By asking for a significant number of samples, say 10,000, we can see that we really do get close to the specified confidence level of successes. Furthermore, we get a confirmation that the standard deviation of the differences that we found is really close to the predicted standard deviation based on the population standard deviations and the sample sizes.

In the folder containing the function scripts for this course create a new directory, copy the model.R file to that directory, rename the file in the new directory, double click on the file to open Rstudio. Then copy all of the text below the line and paste it into your Rstudio editor pane. Then, you can highlight the entire script and run it to use the default values. After that you can go back and change parameters and run the script again to explore the consequences of those changes. 
  
# We look at the 93% confidence interval for the
# difference of two population means when we know
# the standard deviation of each population.

#  first set up some goal populations
pop_one_mean <- 23.2
pop_one_sd <- 5.3
pop_two_mean <- 26.8
pop_two_sd <- 6.2

#  then create the two populations, each with 
#  1000 values

#  First generate an approximate standard normal
pop_one <- rnorm( 1000 )

#  Then get its mean and standard deviation
mu_1 <- mean( pop_one)
#  we want the standard deviation of the population
source("../pop_sd.R")
sd_1 <- pop_sd( pop_one )

#  Then create the distribution we want
pop_one <- ( (pop_one-mu_1)/sd_1 )* pop_one_sd+pop_one_mean

#   finally, verify that we have the right population
mean( pop_one )
pop_sd( pop_one )

#  Now do the same thing for pop_two

#  First generate an approximate standard normal
pop_two <- rnorm( 1000 )

#  Then get its mean and standard deviation
mu_2 <- mean( pop_two)

sd_2 <- pop_sd( pop_two )

#  Then create the distribution we want
pop_two <- ( (pop_two-mu_2)/sd_2 )* pop_two_sd+pop_two_mean

#   finally, verify that we have the right population
mean( pop_two )
pop_sd( pop_two )


#   Now we want to repeat the following process of
#   getting two samples, one from each population,
#   and then generating the 93% confidence interval
#   for the difference of the population means.

#   While we are at this, and because we know what that
#   difference is, we can count the number of times 
#   that the true difference is inside our interval.
#   Furthermore, let us keep track of the observed
#   differences so that later we can compare the
#   standard deviation of those differences to the
#   predicted value


samp_one_size <- 12
samp_two_size <- 18

num_reps <- 100
num_success <- 0
num_fail <- 0
true_diff <- pop_one_mean - pop_two_mean
predicted_sd <- sqrt( pop_one_sd^2/samp_one_size +
                        pop_two_sd^2/samp_two_size )
diff_one_two <- (1:num_reps) # to hold the differences

# since the confidence level is set we can find
#    z value with half the area to its right
z_val <- qnorm( 0.035, lower.tail=FALSE)

for( i in (1:num_reps) )
{  # choose samples from pop one get sample mean
   index_1 <- as.integer( runif( samp_one_size, 1, 1001))
   samp_1 <- pop_one[ index_1 ]
   xbar_1 <- mean( samp_1 )
   # choose samples from pop two get sample mean
   index_2 <- as.integer( runif( samp_two_size, 1, 1001))
   samp_2 <- pop_two[ index_2 ]
   xbar_2 <- mean( samp_2 )
   this_diff <- xbar_1 - xbar_2
   diff_one_two[i] <- this_diff
   
   # get the confidence interval
   ci_low <- this_diff - z_val*predicted_sd
   ci_high <- this_diff + z_val*predicted_sd
   in_ci <- (ci_low <= true_diff ) &&
            ( true_diff <= ci_high )
   if( in_ci )
   { num_success <- num_success+1} else
  
   { num_fail <- num_fail + 1}
}
#  report the number of successes
num_success
#  report the standard deviation of our sample of 
#  differences
sd( diff_one_two )
#  and our predicted value
predicted_sd