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\cfoot{Statistical Sleuth in R: Chapter 2}
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\newcommand{\cran}{\href{http://www.R-project.org/}{CRAN}}

\title{The Statistical Sleuth in R: \\
Chapter 2}

\author{
Linda Loi \and Ruobing Zhang\and Kate Aloisio \and Nicholas J. Horton\thanks{Department of Mathematics and Statistics, Smith College, nhorton@smith.edu}
}

\date{\today}

\begin{document}


\maketitle
\tableofcontents

%\parindent=0pt


<<setup, include=FALSE, cache=FALSE>>=
require(knitr)
opts_chunk$set(
  dev="pdf",
  fig.path="figures/",
        fig.height=3,
        fig.width=4,
        out.width=".47\\textwidth",
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        fig.align="center",
        prompt=TRUE,  # show the prompts; but perhaps we should not do this
        comment=NA    # turn off commenting of ouput (but perhaps we should not do this either
  )
@


<<pvalues, echo=FALSE, message=FALSE>>=
print.pval = function(pval) {
  threshold = 0.0001
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                ifelse(pval > 0.1, paste("p=",round(pval, 2), sep=""),
                       paste("p=", round(pval, 3), sep=""))))
}
@

<<setup2,echo=FALSE,message=FALSE>>=
require(Sleuth3)
require(mosaic)
trellis.par.set(theme=col.mosaic())  # get a better color scheme
set.seed(123)
# this allows for code formatting inline.  Use \Sexpr{'function(x,y)'}, for exmaple.
knit_hooks$set(inline = function(x) {
if (is.numeric(x)) return(knitr:::format_sci(x, 'latex'))
x = as.character(x)
h = knitr:::hilight_source(x, 'latex', list(prompt=FALSE, size='normalsize'))
h = gsub("([_#$%&])", "\\\\\\1", h)
h = gsub('(["\'])', '\\1{}', h)
gsub('^\\\\begin\\{alltt\\}\\s*|\\\\end\\{alltt\\}\\s*$', '', h)
})
showOriginal=FALSE
showNew=TRUE
@

\section{Introduction}

This document is intended to help describe how to undertake analyses introduced as examples in the Third Edition of the \emph{Statistical Sleuth} (2013) by Fred Ramsey and Dan Schafer.
More information about the book can be found at \url{http://www.proaxis.com/~panorama/home.htm}.
This
file as well as the associated \pkg{knitr} reproducible analysis source file can be found at
\url{http://www.math.smith.edu/~nhorton/sleuth3}.


This work leverages initiatives undertaken by Project MOSAIC (\url{http://www.mosaic-web.org}), an NSF-funded effort to improve the teaching of statistics, calculus, science and computing in the undergraduate curriculum. In particular, we utilize the
\pkg{mosaic} package, which was written to simplify the use of R for introductory statistics courses. A short summary of the R needed to teach introductory statistics can be found in the mosaic package vignette (\url{http://cran.r-project.org/web/packages/mosaic/vignettes/MinimalR.pdf}).

To use a package within R, it must be installed (one time), and loaded (each session). The package can be installed using the following command:
<<install_mosaic,eval=FALSE>>=
install.packages('mosaic')               # note the quotation marks
@
Once this is installed, it can be loaded by running the command:
<<load_mosaic,eval=FALSE>>=
require(mosaic)
@
This
needs to be done once per session.

In addition the data files for the \emph{Sleuth} case studies can be accessed by installing the \pkg{Sleuth3} package.
<<install_Sleuth3,eval=FALSE>>=
install.packages('Sleuth3')               # note the quotation marks
@
<<load_Sleuth3,eval=FALSE>>=
require(Sleuth3)
@

We also set some options to improve legibility of graphs and output.
<<eval=TRUE>>=
trellis.par.set(theme=col.mosaic())  # get a better color scheme for lattice
options(digits=3, show.signif.stars=FALSE)
@

The specific goal of this document is to demonstrate how to calculate the quantities described in Chapter 2: Inference Using \emph{t}-Distributions using R.

\section{Evidence Supporting Darwin's Theory of Natural Selection}

Do birds evolve to adapt to their environments? That's the question being addressed by Case Study 2.1 in the \emph{Sleuth}.

\subsection{Statistical summary and graphical display}
We begin by reading the data and summarizing the variables.

<<>>=
summary(case0201)
fav = favstats(Depth ~ Year, data=case0201); fav
@

A total of \Sexpr{nrow(case0201)} subjects are included in the data: \Sexpr{nrow(subset(case0201, Year=="1976"))} are finches that were caught in 1976 and \Sexpr{nrow(subset(case0201, Year=="1978"))} are finches that were caught in 1978.
The following figure replicates Display 2.1 on page 30.

<<>>=
bwplot(Year ~ Depth, data=case0201)
@

<<>>=
densityplot(~ Depth, groups=Year, auto.key=TRUE, data=case0201)
@

The distributions are approximately normally distributed, with some evidence for a long left tail.

\subsection{Inferential procedures (two-sample t-test)}

First, we calculate the pooled SD and the standard error between these two different sample average (page 41, Display 2.8).
<<>>=
# Calculate Pooled SD
n1 = fav["1976", "n"]; n1
n2 = fav["1978", "n"]; n2
s1 = fav["1976", "sd"]; s1
s2 = fav["1978", "sd"]; s2
Sp = sqrt(((n1-1)*(s1)^2+(n2-1)*(s2)^2)/(n1+n2-2)); Sp
# Calculate standard error
SE = Sp*sqrt(1/n1+1/n2); SE
@

So the pooled SD is \Sexpr{round(Sp, 2)} and the standard error is \Sexpr{round(SE, 1)}.

Based on this information, we can construct a 95\% confidence interval (page 43, Display 2.9).

<<>>=
Y1 = fav["1976", "mean"]; Y1
Y2 = fav["1978", "mean"]; Y2
Yd = Y2-Y1; Yd
df = n1+n2-2; df
qt = qt(0.975, df); qt
hw = qt*SE; hw
lower = Yd-hw; lower
upper = Yd+hw; upper
@

So the 95\% confidence interval of the difference between means is (\Sexpr{round(lower, 1)}, \Sexpr{round(upper, 1)})

Now we want to calculate the $t$-statistic and $p$-value (as shown on page 46, Display 2.10).
<<>>=
tstats = (Yd-0)/SE; tstats      # The hypothesis difference=0
onepval = 1-pt(tstats, df); onepval
twopval = 2*onepval; twopval
@

The one-sided $p$-value is approximately \Sexpr{round(onepval, 2)} and the two-sided $p$-value is also approximately \Sexpr{round(twopval, 2)}.

We can get the results of ``Summary of Statistical Findings" (page 29) by using the following code:
<<>>=
t.test(Depth ~ Year, var.equal=TRUE, data=case0201)
confint(lm(Depth ~ Year, data=case0201))
@

\section{Anatomical Abnormalities Associated with Schizophrenia}

Is the area of brain related to the development of schizophrenia? That's the question being addressed by case study 2.2 in the \emph{Sleuth}.

\subsection{Statistical summary and graphical display}
We begin by reading the data and summarizing the variables.

<<>>=
summary(case0202)
@

A total of \Sexpr{nrow(case0202)} subjects are included in the data. There are \Sexpr{nrow(case0202[ "Affected"])} pairs of twins; one of the twins has schizophrenia, and the other does not. So there are \Sexpr{nrow(case0202["Affected"])} affected subjects and \Sexpr{nrow(case0202["Unaffected"])} unaffected subjects.

The difference in area of left hippocampus of these pairs of twins is:
<<transform>>=
case0202 = transform(case0202, DIFF = Unaffected - Affected)
favstats(~ DIFF, data=case0202)
@

This matches the results on page 31, Display 2.2.

<<>>=
densityplot(~ DIFF, data=case0202)
@

\subsection{Inferential procedures (two-sample t-test)}

We want to calculate the paired t-test and 95\% confidence interval.

<<>>=
# Calculate t-statistics
difmean = mean(~ DIFF, data=case0202); difmean
difsd = sd(~ DIFF, data=case0202); difsd
difn = nrow(case0202); difn
difSE = difsd/sqrt(difn); difSE
tscore = (difmean-0)/difSE; tscore         # hypothesis difference=0
twopvalue = 2*(1-pt(tscore, difn-1)); twopvalue
# Construct confidence interval
tstar = qt(0.975, difn-1); tstar
schizolower = difmean - tstar*difSE; schizolower
schizoupper = difmean + tstar*difSE; schizoupper
@

So the two-sided $p$-value is approximately \Sexpr{round(twopvalue, 3)} and the 95\% confidence interval is (\Sexpr{round(schizolower, 2)}, \Sexpr{round(schizoupper, 2)}).

We can also get the results displayed on page 32 by conducting a paired $t$-test:

<<>>=
with(case0202, t.test(Unaffected, Affected, paired=TRUE))
@

\end{document}



