RESEARCH STUDIES
The studies that have been produced over the years have run
the gambit in type, parameter, and result. A quick review of the available published data
would leave one with the impression that the spectrographic method of voice identification
was only somewhat more accurate than flipping a coin. The diversity of the relatively low
number of studies and the range of results has only added to the confusion as to the
reliability and validity of this method of identification. When one takes the time and
expends the effort to analyze the studies in this field, a very different conclusion
becomes evident. When the individual parameters of the studies are taken into account, who
was being evaluated, what information was given to the examiner to assess, and what
limitations were placed on the examiner's conclusions, a much clearer picture of the
accuracy of the spectrographic voice identification method develops. The picture is not
one of a marginally accurate technique but rather a picture that clearly shows that a
properly trained and experienced examiner, adhering to internationally accepted standards
will produce a highly accurate result. The studies also show that as the level of training
diminishes and/or the conclusions an examiner may reach are artificially limited, the
error rate goes up dramatically.
The training for accurately performing the spectrographic
voice identification method has been established as requiring completion of (1) a formal
course of study, usually 2 to 4 weeks duration, in the basics of spectrographic analysis,
(2) two years of study completing 100 voice comparison cases, usually in a one-to-one
relationship with a recognized expert, (3) examination by a board of experts in the field
of spectrographic voice identification analysis.
For the most accurate results from the spectrographic voice
identification method, a professional examiner (1) will require the original recordings or
the best quality re-recordings if the original is not available; (2) will perform a
critical aural review of the suspect and known recordings; (3) will produce sound
spectrograms of the comparable words and phrases; (4) will produce a comparison recording
juxtaposing the known and unknown speech samples; (5) will evaluate the evidence and
classify the results into one of five standard categories [ 1 - positive identification, 2
- probable identification, 3. - positive elimination, 4 - probable elimination, and 5 - no
decision]. The final decision is reached through a combined process of aural and visual
examination.
It is important to remember that the spectrographic method
of voice identification is a process that interweaves the visual analysis of the sound
spectrograms with the critical aural examination of the sounds being viewed. Taking the
results from all of the studies produced shows that if the examiner's ability to analyze
both the graphic representations of the voice and the aural cues found in the recordings
is limited or restricted, accuracy suffers. Likewise, the amount of training has a direct
bearing on the level of accuracy of the results.
In a survey of 18 studies151 of the accuracy of the
spectrographic voice identification method, the results fall into two categories; those
with proper training, using standard procedures produce very accurate results, whereas
those with inadequate training, using limited analysis methods, produce inaccurate
results.
In a study152 in 1975 authored by Lt. L. Smrkovski of the
Voice Identification Unit of the Michigan State police, error rates in voice
identification analysis comparisons, based on three levels of training and experience,
were evaluated. The following table summarizes the results of that study.
Error type Novice Trainee Professional
False Ident. 5.0% 0.0% 0.0%
False Elim. 25.0% 0.0% 0.0%
No Decision 2.5% 2.5% 7.5%
Lt. Smrkovski's results show that proper training is
essential. The fact that his results show a higher no decision rate among the professional
examiners than the trainee examiners may indicate that the professional is a bit more
cautious in his analysis than the trainee.
Mark Greenwald, in his 1979 thesis153 for his M.A. degree
at Michigan State University, studied the performance of three professional examiners
(each with eight years experience) and five trainees (each with less than two years
experience) using standard spectrographic voice identification methods (visual and aural)
and result classifications. Greenwald found that the professional examiners produced no
errors when using full frequency bandwidth recordings. When the frequency band width was
restricted, the professional examiners still produced no errors, but did increase their
percentage of no decision classifications. Greenwald also found that the training level
was an important factor and that the trainees in this study had an error rate of 6.1% for
false identifications in the restricted frequency bandwidth trials.
In 1986, the Federal Bureau of Investigation published a
survey of two thousand voice identification comparisons made by FBI examiners154. This
survey was based on 2000 forensic comparisons completed over a period of fifteen years,
under actual law enforcement conditions, by FBI examiners155.
The examiners had a minimum of two years experience,
completed over 100 actual cases, completed a basic two week training course and received
formal approval by other trained examiners.156
The results of the survey are depicted in the chart157
below.
DECISIONS NUMBER PERCENT(%)
No or low confidence 1304 65.2
Eliminations 378 18.9
Identifications 318 15.9
ERRORS
False eliminations 2 0.53
False identification 1 0.31
The FBI results are consistent with the Smrkovski study in
that properly trained examiners, utilizing the full range of procedures, produce quite
accurate results.
By way of contrast, the 1976 study158 by Alan Reich used
four speech science graduate students with previous experience with speech spectrograms
(but untrained in spectrographic voice identification analysis) to examine, using visual
comparison only, nine excerpted words. This study produced an accuracy rate in the
undisguised trials of 56.67%. When disguise was introduced into this study paradigm the
accuracy rate decreased significantly.
Taken as a whole the 18 studies support the conclusion that
accurate results will be obtained only through the combined use of the aural and visual
components of the spectrographic voice identification method as performed by a properly
trained examiner adhering to the established standards. Those studies with poor accuracy
results are important in that they demonstrate the weaknesses of improperly performed
examinations that do not adhere to the internationally accepted professional standards.
A large part of the debate over the admissibility of
spectrographic voice identification analysis in the courts appears due to the fact that
the parameters of these studies have not adequately been demonstrated to the courts in the
necessary detail which would allow the courts to examine the overall meaning of these
studies. Many of these studies look at only one or two aspects of the spectrographic voice
identification method. Frequently the results of these restricted scope studies have been
misapplied to the entire spectrographic voice identification method resulting in
inaccurate information being used as the basis for deciding the admissibility of
spectrographic voice identification analysis. It is important to provide an accurate
picture of all the studies so the courts will have the foundational information necessary
to make an informed decision regarding the admissibility of spectrographic voice
identification analysis.