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\author{Rafael Irizarry and Leslie Cope}
\begin{document}
\title{Bioconductor Expression Assessment Tool for Affymetrix Oligonucleotide Arrays (affycomp)}

\maketitle
\tableofcontents
\section{Introduction}
<<echo=F,results=hide>>=
library(affycomp)
@

\section{Introduction}
The defining feature of 
oligonucleotide expression arrays is the use of several probes to
assay each targeted transcript.  This is  
a bonanza for the statistical geneticist, offering a great opportunity
to create probeset summaries with
specific characteristics. There are now several methods available
for summarizing probe level data from the popular Affymetrix
GeneChips. It is  harder to identify the method best suited to a given
inquiry.  This package provides a {\it graphical tool}  for summaries of
Affymetrix probe level data.  Plots and summary statistics offer a
picture of how an expression measure performs in several important
areas.  This picture facilitates the comparison of competing
expression measures and the selection of  methods suitable for a
specific investigation. The key is a  benchmark dataset consisting of
a dilution study and a spike-in study.  Because  the {\it truth} is 
known for this data, it is possible to identify statistical features
of the data for which the expected outcome is known in advance.
Those features highlighted in our suite of graphs are justified by
questions of biological interest, and motivated by the presence of
appropriate data. 

The benchmark data used is freely
available to the public.  The assessment data sets are the following:
For the  dilution study by
\url{http://qolotus02.genelogic.com/datasets.nsf/}{GeneLogic}, two 
sources of cRNA, human liver tissue and central nervous system cell
line (CNS), were hybridized  
to human arrays (HG-U95Av2) in a range of dilutions and proportions
\cite{RMA}. 

For the spike-in study, different cRNA fragments were added to the 
hybridization mixture of the arrays at different picoMolar
concentrations. The cRNAs were  
spiked-in at a different concentration on each array (apart from
replicates) arranged in a cyclic Latin square design with each
concentration appearing once in each row and column. All arrays had a
common background cRNA. The data can be obtained form
\url{http://www.affymetrix.com/analysis/download_center2.affx}.


Two phenoData objects are included in the package that give more details:  
\verb+dilution.phenodata+ and \verb+spikein.phenodata+.

\section{What's new in version 1.2?}
A new sets of assessment has been added. The wrapper is called 
\verb+assessSpikeIn2+ and one can send it a exprSet with the spikein as 
\verb+assessSpikeIn+ but it only uses culumnes 1,...,13,17 and 21,...,33,37. 
So one only needs to get expression measures for these 28 arrays if one plans 
to only use \verb+assessSpikeIn2+. The engine functions are \verb+assessLS+, 
\verb+assessSpikeInSD+, and \verb+assessMA2+.
 
All spike in related assessments now work with both the HGU95A (this is the one used in version 1.1) and HGU133A. The function \verb+read.newspikein+ will 
read the HGU133A expression values. 

\section{The Image Report}
If you have a file named \verb+dilfilename.csv+ with the dilution
expression values and  \verb+sifilename.csv+ with the spikein
expression measures, say RMA, you can easily obtain the graphs and
summary statistics in the image report: 

\begin{Sinput}
R> library(affycomp) ##load the affy package
R> d <- read.dilution("dilfilename.csv")
R> s <- read.spikein("sifilename.csv")
R> rma.assessment <- affycomp(d,s,method.name="RMA")
\end{Sinput}

\verb+res+ will have the necessary information to recreate the graphs
without having to wait for the assessment. See below.


\verb+affycomp+ is a wrapper for other \verb+assessAll+ and
\verb+affycompTable+. Here are some examples based

<<>>=
data(mas5.assessment)
data(rma.assessment)
@ 

were created using the function \verb+assessAll+ on
\verb+exprSet+s created using the RMA and MAS 5.0 methods. 
These are lists of lists. 

<<>>=
names(mas5.assessment)
@ 

Each component is the result of a specific assessment. The names tell
us what they are for. \verb+Dilution+ are the assessment based on the
dilution data and can be used to create Figures 2, 3, and
4b. \verb+MA+ has the necessary information for the MA plot or Figure
1. \verb+Signal+ has the necessary information to create Figure
4a. \verb+FC+ has assessments related to fold change and can be used
to create Figures 5a, 6a, and 6b. Finally \verb+FC2+ has the necessary
information to create Figure 5b. The captions for these Figures will
give you an idea of what they are for.

There are two kinds of plots the basic and the comparative. In the
basic plots based on the expression measure being assessed are
shown. In the comparative plots, the expression measure is compared to
other measures, MAS 5.0 by default. Table are also automatically
created with assessment statistics. Finally, a simple assessment of
standard error estimates can be done. These are described in the
following subsections.

\subsection{Basic Plots}

You can use \verb+affycompPlot+ which will automatically know what to do
\begin{figure}[htbp]
\begin{center}
<<fig=TRUE>>=
affycompPlot(mas5.assessment$MA)
@
\end{center}
{Figure 1: The MA plot shows log fold change as a function of
    mean log expression level.  A set of 14 arrays representing a single
    experiment from the Affymetrix spike-in data are used for this plot.
    A total of 13 sets of fold changes are generated by comparing the
    first array in the set to each of the others. Genes are
    symbolized by numbers representing the nominal $\log_2$ fold change
    for the gene.  Non-differentially expressed genes with observed fold
    changes larger than 2 are plotted in red. All other probesets are
    represented with black dots.}
\end{figure}

Or you can use the auxiliary figure functions that will need to have a
specific assessment list
\begin{figure}[htbp]
\begin{center}
<<fig=TRUE>>=
affycomp.figure2(mas5.assessment$Dilution)
@
\end{center}
  {Figure 2:   For each gene,
  and each experimental condition, we calculate the mean log
  expression and the observed standard deviation across   5
  replicates. The
  resulting scatterplot is smoothed to generate a single curve
  representing mean standard deviation as a function of mean log
  expression.}
\end{figure}


\begin{figure}[htbp]
\begin{center}
<<fig=TRUE>>=
affycomp.figure3(mas5.assessment$Dilution)
@
\end{center}
{Figure 3: This plot, using the GeneLogic dilution data,
  shows the sensitivity of fold change calculations to total RNA
  abundance.  Average log fold-changes between liver and CNS for the
  lowest concentration and the   highest in the dilution data set are
  computed.  Orange and red color is used to denote genes with
$M_{6g}-M_{1g}$ bigger than $\log_2(2)$ and $\log_2(3)$
respectively. The rest are denoted with black.}
\end{figure}

\begin{figure}[htbp]
\begin{center}
<<fig=TRUE>>=
par(mfrow=c(2,1))
affycomp.figure4a(mas5.assessment$Signal)
affycomp.figure4b(mas5.assessment$Dilution)
@
\end{center}
{Figure 4: a) Average observed $log_2$ intensity plotted
    against nominal $log_2$ concentration for each spiked-in gene for
    all arrays in  Affymetrix spike-In experiment. b) For the GeneLogic
    dilution data, log expression values are regressed against their
    log nominal concentration. The slope estimates are plotted against
    average log intensity across all concentrations.}
\end{figure}

\begin{figure}[htbp]
\begin{center}
<<fig=TRUE>>=
par(mfrow=c(2,1))
affycomp.figure5a(mas5.assessment$FC)
affycomp.figure5b(mas5.assessment$FC)
@
\end{center}
  {Figure 5: A typical identification rule for differential
      expression filters
      genes with fold change exceeding a given threshold.
      This figure shows average ROC curves  which offer a graphical
      representation of both specificity and sensitivity for such a
      detection rule. a) Average ROC curves based on comparisons with
      nominal fold changes ranging from 2 to 4096. b) As a) but with
      nominal fold changes equal to 2.}
\end{figure}

\begin{figure}[htbp]
\begin{center}
<<fig=TRUE>>=
par(mfrow=c(2,1))
affycomp.figure6a(mas5.assessment$FC)
affycomp.figure6b(mas5.assessment$FC)
@
\end{center}
  {Figure 6: a) Observed log fold changes plotted against
    nominal log fold changes. The dashed lines represent highest, 25th
    highest, 100th  highest, 25th percentile, 75th percentile,
    smallest 100th, smallest 25th, and smallest log fold change for
    the genes that were not differentially expressed. b) Like a) but
    the observed fold changes were calculated for spiked in genes with
    nominal concentrations no higher than 2pM.}
\end{figure}


\subsection{Comparative Plots}

You can use \verb+affycompPlot+ which will automatically know what to do
\begin{figure}[htbp]
\begin{center}
<<fig=TRUE>>=
par(mfrow=c(2,1))
affycompPlot(mas5.assessment$MA,rma.assessment$MA)
@
\end{center}
{Figure 1: The MA plot shows log fold change as a function of
    mean log expression level.  A set of 14 arrays representing a single
    experiment from the Affymetrix spike-in data are used for this plot.
    A total of 13 sets of fold changes are generated by comparing the
    first array in the set to each of the others. Genes are
    symbolized by numbers representing the nominal $\log_2$ fold change
    for the gene.  Non-differentially expressed genes with observed fold
    changes larger than 2 are plotted in red. All other probesets are
    represented with black dots.}
\end{figure}

Or you can use the auxiliary figure functions that will need to have a
specific assessment list
\begin{figure}[htbp]
\begin{center}
<<fig=TRUE>>=
affycomp.compfig2(list(mas5.assessment$Dilution,rma.assessment$Dilution),
                  method.names=c("MAS 5.0","RMA"))
@
\end{center}
  {Figure 2:   For each gene,
  and each experimental condition, we calculate the mean log
  expression and the observed standard deviation across   5
  replicates. The
  resulting scatterplot is smoothed to generate a single curve
  representing mean standard deviation as a function of mean log
  expression.}
\end{figure}

\begin{figure}[htbp]
\begin{center}
<<fig=TRUE>>=
affycomp.compfig3(list(mas5.assessment$Dilution,rma.assessment$Dilution),
                  method.names=c("MAS 5.0","RMA"))
@
\end{center}
{Figure 3: This plot, using the GeneLogic dilution data,
  shows the sensitivity of fold change calculations to total RNA
  abundance.  Average log fold-changes between liver and CNS for the
  lowest concentration and the   highest in the dilution data set are
  computed.  Orange and red color is used to denote genes with
$M_{6g}-M_{1g}$ bigger than $\log_2(2)$ and $\log_2(3)$
respectively. The rest are denoted with black.}
\end{figure}

\begin{figure}[htbp]
\begin{center}
<<fig=TRUE>>=
par(mfrow=c(2,1))
affycomp.compfig4a(list(mas5.assessment$Signal,rma.assessment$Signal),
                  method.names=c("MAS 5.0","RMA"))
affycomp.compfig4b(list(mas5.assessment$Dilution,rma.assessment$Dilution),
                  method.names=c("MAS 5.0","RMA"))
@
\end{center}
  {Figure 4: a) Average observed $log_2$ intensity plotted
    against nominal $log_2$ concentration for each spiked-in gene for
    all arrays in  Affymetrix spike-In experiment. b) For the GeneLogic
    dilution data, log expression values are regressed against their
    log nominal concentration. The slope estimates are plotted against
    average log intensity across all concentrations.}
\end{figure}

\begin{figure}[htbp]
\begin{center}
<<fig=TRUE>>=
par(mfrow=c(2,1))
affycomp.compfig5a(list(mas5.assessment$FC,rma.assessment$FC),
                  method.names=c("MAS 5.0","RMA"))
affycomp.compfig5b(list(mas5.assessment$FC2,rma.assessment$FC2),
                  method.names=c("MAS 5.0","RMA"))
@
\end{center}
  {Figure 5: A typical identification rule for differential
      expression filters
      genes with fold change exceeding a given threshold.
      This figure shows average ROC curves  which offer a graphical
      representation of both specificity and sensitivity for such a
      detection rule. a) Average ROC curves based on comparisons with
      nominal fold changes ranging from 2 to 4096. b) As a) but with
      nominal fold changes equal to 2.}
\end{figure}

\begin{figure}[htbp]
\begin{center}
<<fig=TRUE>>=
par(mfrow=c(2,2))
affycomp.figure6a(mas5.assessment$FC,main="a) MAS 5.0",ylim=c(-12,12))
affycomp.figure6a(rma.assessment$FC,main="a) RMA",ylim=c(-12,12))
affycomp.figure6b(mas5.assessment$FC,main="b) MAS 5.0",ylim=c(-6,6))
affycomp.figure6b(rma.assessment$FC,main="b) RMA",ylim=c(-6,6))
@
\end{center}
  {Figure 6: a) Observed log fold changes plotted against
    nominal log fold changes. The dashed lines represent highest, 25th
    highest, 100th  highest, 25th percentile, 75th percentile,
    smallest 100th, smallest 25th, and smallest log fold change for
    the genes that were not differentially expressed. b) Like a) but
    the observed fold changes were calculated for spiked in genes with
    nominal concentrations no higher than 2pM.}
\end{figure}

\subsection{Tables}
The function \verb+tableAll+ returns a matrix with assessment
statistics. Once the assessment function are run all one needs to type
is 


<<>>=
data(rma.assessment)
data(mas5.assessment)
tableAll(rma.assessment,mas5.assessment)
@ 

The function \verb+affycompTable+ make a minimal table (that is also
more informative).

<<>>=
affycompTable(rma.assessment,mas5.assessment)
@ 

\subsection{Standard deviation assessment}
The package also contains a simple tool to assess standard error
estimates. For this to work the \verb+exprSet+ object used for the
assessment must have standard error estimates for the dilution data.
We include two examples in the package.

<<>>=
data(rma.sd.assessment)
data(lw.sd.assessment)
tableAll(rma.sd.assessment,lw.sd.assessment)
@ 

For the SD assessment, there are also comparison plots as well as
simple plots. 

\begin{figure}[htbp]
\begin{center}
<<fig=TRUE>>=
affycompPlot(lw.sd.assessment,rma.sd.assessment)
@
\end{center}
{Figure 7:  Using the replicates from the dilution data, we calculate the mean
  predicted variance for each gene, tissue and dilution by squaring
  the estimated standard error.  The usual sample variance  $s^2_{tdg}
  = \sum_r (y_{tdrg} - y_{td\cdot g})^2/4$ are calculated as
  well. These boxplots are of the log ratios of the predicted and  
  observed variance.
}
\end{figure}

There are also comparison plots as well as simple plots.
\begin{figure}[htbp]
\begin{center}
<<fig=TRUE>>=
affycompPlot(lw.sd.assessment)
@
\end{center}
{Figure 7:  Using the replicates from the dilution data, we calculate the mean
  predicted variance for each gene, tissue and dilution by squaring
  the estimated standard error.  The usual sample variance  $s^2_{tdg}
  = \sum_r (y_{tdrg} - y_{td\cdot g})^2/4$ are calculated as
  well. These boxplots are of the log ratios of the predicted and  
  observed variance.
}
\end{figure}

Enjoy!
\end{document}
