Sleuth3 (version 0.1-8)

case0301: Cloud Seeding

Description

Does dropping silver iodide onto clouds increase the amount of rainfall they produce? In a randomized experiment, researchers measured the volume of rainfall in a target area (in acre-feet) on 26 suitable days in which the clouds were seeded and on 26 suitble days in which the clouds were not seeded.

Usage

case0301

source

Ramsey, F.L. and Schafer, D.W. (2013). The Statistical Sleuth: A Course in Methods of Data Analysis (3rd ed), Cengage Learning.

References

Simpson, J., Olsen, A., and Eden, J. (1975). A Bayesian Analysis of a Multiplicative Treatment Effect in Weather Modification. Technometrics 17: 161--166.

Examples

attach(case0301) 
str(case0301) #Seeded is level 1 of Treatment (it's first alphabetically)

boxplot(Rainfall ~ Treatment) 
boxplot(log(Rainfall) ~ Treatment) # Boxplots of natural logs of Rainfall 

t.test(log(Rainfall) ~ Treatment, var.equal=TRUE,
  alternative="greater") # 1-sided t-test; alternative: level 1 mean is greater
        

myTest <- t.test(log(Rainfall) ~ Treatment,  var.equal=TRUE,
  alternative="two.sided") # 2-sided alternative to get confidence interval
exp(myTest$est[1] - myTest$est[2])  # Back-transform estimate on log scale 
exp(myTest$conf) # Back transform endpoints of confidence interval 

boxplot(log(Rainfall) ~ Treatment, 
  ylab="Log of Rainfall Volume in Target Area (Acre Feet)", 
  names=c("On 26 Seeded Days", "On 26 Unseeded Days"), 
  main="Distributions of Rainfalls from Cloud Seeding Experiment") 

## POLISHED BOXPLOTS FOR PRESENTATION:
opar <- par(no.readonly=TRUE)  # Store device graphics parameters
par(mar=c(4,4,4,4))   # Change margins to allow more space on right
boxplot(log(Rainfall) ~ Treatment, ylab="Log Rainfall (Acre-Feet)",
  names=c("on 26 seeded days","on 26 unseeded days"), 
  main="Boxplots of Rainfall on Log Scale", col="green", boxlwd=2,
  medlwd=2, whisklty=1, whisklwd=2, staplewex=.2, staplelwd=2,
  outlwd=2, outpch=21, outbg="green", outcex=1.5      )        
myTicks <- c(1,5,10,100,500,1000,2000,3000) # some tick marks for original scale 
axis(4, at=log(myTicks), label=myTicks)   # Add original-scale axis on right    
mtext("Rainfall (Acre Feet)", side=4, line=2.5) # Add right-side axis label 
par(opar)  # Restore previous graphics parameter settings

detach(case0301)