nbest-optimize.1

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.\" $Id: nbest-optimize.1,v 1.19 2006/11/20 20:39:15 stolcke Exp $
.TH nbest-optimize 1 "$Date: 2006/11/20 20:39:15 $" "SRILM Tools"
.SH NAME
nbest-optimize \- optimize score combination for N-best word error minimization
.SH SYNOPSIS
.B nbest-optimize
[\c
.BR \-help ]
option
\&...
[
.I scoredir
\&...
]
.SH DESCRIPTION
.B nbest-optimize
reads a set of N-best lists, additional score files, and corresponding 
reference transcripts and optimizes the score combination weights
so as to minimize the word error of a classifier that performs
word-level posterior probability maximization.
The optimized weights are meant to be used with
.BR nbest-lattice (1)
and the
.B \-use-mesh 
option,
or the 
.B nbest-rover
script (see
.BR nbest-scripts (1)).
.B nbest-optimize
determines both the best relative weighting of knowledge source scores
and the optimal 
.B \-posterior-scale
parameter that controls the peakedness of the posterior distribution.
.PP
The optimization is performed by gradient descent on a smoothed (sigmoidal)
approximation of the true 0/1 word error function (Katagiri et al. 1990).
Therefore, the result can only be expected to be a
.I local
minimum of the error surface.
(A more global search can be attempted by specifying different starting
points.)
Another approximation is that the error function is computed assuming a fixed
multiple alignment of all N-best hypotheses and the reference string,
which tends to slightly overestimate the true pairwise error between any 
single hypothesis and the reference.
.PP
An alternative search strategy uses a simplex-based "Amoeba" search on
the (non-smoothed) word error function (Press et al. 1988).
The search is restarted multiple times to avoid local minima.
.PP
Alternatively,
.B nbest-optimize
can also optimize weights for a standard, 1-best hypothesis rescoring that
selects entire (sentence) hypotheses
.RB ( \-1best
option).
In this mode sentence-level error counts may be read from external files,
or computed on the fly from the reference strings.
The weights obtained are meant to be used for N-best list rescoring with
.B rescore-reweight
(see 
.BR nbest-scripts (1)).
.SH OPTIONS
.PP
Each filename argument can be an ASCII file, or a 
compressed file (name ending in .Z or .gz), or ``-'' to indicate
stdin/stdout.
.TP
.B \-help
Print option summary.
.TP
.B \-version
Print version information.
.TP
.BI \-debug " level"
Controls the amount of output (the higher the
.IR level ,
the more).
At level 1, error statistics at each iteration are printed.
At level 2, word alignments are printed.
At level 3, full score matrix is printed.
At level 4, detailed information about word hypothesis ranking is printed
for each training iteration and sample.
.TP
.BI \-nbest-files " file-list"
Specifies the set of N-best files as a list of filenames.
Three sets of standard scores are extracted from the N-best files:
the acoustic model score, the language model score, and the number of 
words (for insertion penalty computation).
See 
.BR nbest-format (5)
for details.
.TP
.BI \-refs " references"
Specifies the reference transcripts.
Each line in 
.I references
must contain the sentence ID (the last component in the N-best filename
path, minus any suffixes) followed by zero or more reference words.
.TP
.BI \-insertion-weight " W"
Weight insertion errors by a factor 
.IR W .
This may be useful to optimize for keyword spotting tasks where
insertions have a cost different from deletion and substitution errors.
.TP
.BI \-word-weights " file"
Read a table of words and weights from
.IR file .
Each word error is weighted according to the word-specific weight.
The default weight is 1, and used if a word has no specified weight.
Also, when this option is used, substitution errors are counted 
as the sum of a deletion and an insertion error, as opposed to counting
as 1 error as in traditional word error computation.
.TP
.B \-1best
Select optimization for standard sentence-level hypothesis selection.
.TP
.B \-1best-first
Optimized first using 
.B \-1best
mode, then switch to full optimization.
This is an effective way to quickly bring the score weights near an
optimal point, and then fine-tune them jointly with the posterior scale
parameter.
.TP
.BI \-errors " dir"
In 1-best mode, optimize for error counts that are stored in separate files
in directory
.IR dir .
Each N-best list must have a matching error counts file of the same 
basename in 
.IR dir .
Each file contains 7 columns of numbers in the format
.br
	wcr wer nsub ndel nins nerr nw
.br
Only the last two columns (number of errors and words, respectively) are used.
.br
If this option is omitted, errors will be computed from the N-best hypotheses
and the reference transcripts.
.TP
.BI \-max-nbest " n"
Limits the number of hypotheses read from each N-best list to the first
.IR n .
.TP
.BI \-rescore-lmw " lmw"
Sets the language model weight used in combining the language model log
probabilities with acoustic log probabilities.
This is used to compute initial aggregate hypotheses scores.
.TP
.BI \-rescore-wtw " wtw"
Sets the word transition weight used to weight the number of words relative to
the acoustic log probabilities.
This is used to compute initial aggregate hypotheses scores.
.TP
.BI \-posterior-scale " scale"
Initial value for scaling log posteriors.
The total weighted log score is divided by 
.I scale
when computing normalized posterior probabilities.
This controls the peakedness of the posterior distribution. 
The default value is whatever was chosen for 
.BR \-rescore-lmw , 
so that language model scores are scaled to have weight 1,
and acoustic scores have weight 1/\fIlmw\fP.
.TP
.B \-combine-linear
Compute aggregate scores by linear combination, rather than log-linear
combination.
(This is appropriate if the input scores represent log-posterior probabilities.)
.TP
.B \-non-negative
Constrain search to non-negative weight values.
.TP
.BI \-vocab " file"
Read the N-best list vocabulary from 
.IR file .
This option is mostly redundant since words found in the N-best input
are implicitly added to the vocabulary.
.TP
.B \-tolower
Map vocabulary to lowercase, eliminating case distinctions.
.TP
.B \-multiwords
Split multiwords (words joined by '_') into their components when reading
N-best lists.
.TP
.B \-no-reorder
Do not reorder the hypotheses for alignment, and start the alignment with
the reference words.
The default is to first align hypotheses by order of decreasing scores
(according to the initial score weighting) and then the reference,
which is more compatible with how 
.BR nbest-lattice (1)
operates.
.TP
.BI \-noise " noise-tag"
Designate
.I noise-tag
as a vocabulary item that is to be ignored in aligning hypotheses with
each other (the same as the -pau- word).
This is typically used to identify a noise marker.
.TP
.BI \-noise-vocab " file"
Read several noise tags from
.IR file ,
instead of, or in addition to, the single noise tag specified by
.BR \-noise .
.TP
.BR \-hidden-vocab " file"
Read a subvocabulary from
.I file
and constrain word alignments to only group those words that are either all
in or outside the subvocabulary.
This may be used to keep ``hidden event'' tags from aligning with
regular words.
.TP
.BI \-init-lambdas " 'w1 w2 ...'"
Initialize the score weights to the values specified
(zeros are filled in for missing values).
The default is to set the initial acoustic model weight to 1,
the language model weight from
.BR \-rescore-lmw ,
the word transition weight from
.BR \-rescore-wtw ,
and all remaining weights to zero initially.
Prefixing a value with an equal sign (`=')
holds the value constant during optimization.
(All values should be enclosed in quotes to form a single command-line
argument.)
.br
Hypotheses are aligned using the initial weights; thus, it makes sense
to reoptimize with initial weights from a previous optimization in order
to obtain alignments closer to the optimimum.
.TP
.BI \-alpha " a"
Controls the error function smoothness; 
the sigmoid slope parameter is set to
.IR a .
.TP
.BI \-epsilon " e"
The step-size used in gradient descent (the multiple of the gradient vector).
.TP
.BI \-min-loss " x"
Sets the loss function for a sample effectively to zero when its value falls
below 
.IR x .
.TP
.BI \-max-delta " d"
Ignores the contribution of a sample to the gradient if the derivative
exceeds
.IR d .
This helps avoid numerical problems.
.TP
.BI \-maxiters " m"
Stops optimization after 
.I m
iterations.
In Amoeba search, this limits the total number of points in the parameter space
that are evaluated.
.TP
.BR \-max-bad-iters " n"
Stops optimization after 
.I n
iterations during which the actual (non-smoothed) error has not decreased.
.TP
.BR \-max-amoeba-restarts " r"
Perform only up to
.I r
repeated Amoeba searches.
The default is to search until 
.I D
searches give the same results, where
.I D 
is the dimensionality of the problem.
.TP
.BI \-max-time " T"
Abort search if new lower-error point isn't found in 
.I T
seconds.
.TP
.BI \-epsilon-stepdown " s"
.TP
.BI \-min-epsilon " m"
If 
.I s
is a value greater than zero, the learning rate will be multiplied by 
.I s
every time the error does not decrease after a number of iterations
specified by
.BR \-max-bad-iters .
Training stops when the learning rate falls below
.I m
in this manner.
.TP
.BI \-converge " x"
Stops optimization when the (smoothed) loss function changes relatively by less 
than 
.I x
from one iteration to the next.
.TP
.B \-quickprop
Use the approximate second-order method known as "QuickProp" (Fahlman 1989).
.TP
.BI \-init-amoeba-simplex " 's1 s2 ...'"
Defines the step size for the initial Amoeba simplex.
One value for each non-fixed search dimension should be specified,
plus optionally a value for the posterior scaling parameter
(which is searched as an added dimension).
.TP
.BI \-print-hyps " file"
Write the best word hypotheses to 
.I file
after optimization.
.TP
.BI \-write-rover-control " file"
Writes a control file for 
.B nbest-rover
to 
.IR file ,
reflecting the names of the input directories and the optimized parameter
values.
The format of
.I file
is described in
.BR nbest-scripts (1).
The file is rewritten for each new minimal error weight combination found.
.TP
.B \--
Signals the end of options, such that following command-line arguments are 
interpreted as additional scorefiles even if they start with `-'.
.TP
.IR scoredir ...
Any additional arguments name directories containing further score files.
In each directory, there must exist one file named after the sentence 
ID it corresponds to (the file may also end in ``.gz'' and contain compressed
data).
The total number of score dimensions is thus 3 (for the standard scores from
the N-best list) plus the number of additional score directories specified.
.SH "SEE ALSO"
nbest-lattice(1), nbest-scripts(1), nbest-format(5).
.br
S. Katagiri, C.H. Lee, & B.-H. Juang, "A Generalized Probabilistic Descent
Method", in
\fIProceedings of the Acoustical Society of Japan, Fall Meeting\fP,
pp. 141-142, 1990.
.br
S. E. Fahlman, "Faster-Learning Variations on Back-Propagation: An
Empirical Study", in D. Touretzky, G. Hinton, & T. Sejnowski (eds.), 
\fIProceedings of the 1988 Connectionist Models Summer School\fP, pp. 38-51,
Morgan Kaufmann, 1989.
.br
W. H. Press, B. P. Flannery, S. A. Teukolsky, & W. T. Vetterling,
\fINumerical Recipes in C: The Art of Scientific Computing\fP,
Cambridge University Press, 1988.
.br
.SH BUGS
Gradient-based optimization is not supported (yet) in 1-best mode
or in conjunction with the 
.B \-combine-linear
or 
.B \-non-negative
options;
use simplex-search optimization instead.
.br
The N-best directory in the control file output by
.B \-write-rover-control
is inferred from the
first N-best filename specified with
.BR \-nbest-files ,
and will therefore only work if all N-best lists are placed in the same
directory.
.PP
The
.B \-insertion-weight
and 
.B \-word-weights
options only affect the word error computation, not the construction 
of hypothesis alignments. 
Also, they only apply to sausage-based, not 1-best error optimization.
(1-best errors may be explicitly specified using the 
.B \-errors
option).
.SH AUTHORS
Andreas Stolcke <[e-mail]>
.br
Dimitra Vergyri <[e-mail]>
.br
Copyright 2000\-2006 SRI International