Comparing Expert Fingerprint Examiners and Novices on a Visual Working Memory Task: What Makes Experts So Good?
Technical Report
Thomas Busey, Ph.D.
Indiana University, Bloomington
busey@indiana.edu
The following is a summary of an experiment that addresses the information used by novice and fingerprint experts when
performing a tasks that abstracts many of the relevant factors from real fingerprint comparisons. While there remain important
methodological differences between a real comparison and the present study, the results bear on the nature of the differences
between experts examiners and novices.
Methods
The goal of the experiment is to determine what information novices and fingerprint examiners might use when comparing
fingerprint features. In the experiment, a single feature was presented for 1400 ms (or 1.4 seconds):
It was then replaced by a mask, which was on the screen for either 200 ms or 5200 ms:
and then two features were shown:
The subject then indicated which of the two test prints corresponded to the studied print. In this example, the feature on the left
matches the studied print. The images remained on the screen until the observer responded, which was not time-limited.
Note that we have altered the orientation of the images, and slightly changed the brightness levels to prevent observers from using
low-level cues to make a match. These types of cues would be irrelevant in an inked print/latent print comparison. We also removed
images that had artifacts such as lint flakes that were specific to the inking procedure and would not also be present in a latent print
(such as a scar). These procedures were designed to make the testing conditions rely on many of the same sources of information
that an examiner would use while matching latent to inked prints. However, the testing procedures (limited viewing time) were
designed to probe the results of perceptual expertise rather than mimic actual viewing conditions, which usually are not timelimited.
Three different factors were included in the experiment. First, the mask could be shown for a short delay (a fifth of a second) or a
long delay (5200 ms). Comparison across the delays will tell us whether, for example, experts are better at holding the features in
memory.
The second factor was the inclusion of visual noise at the test images. Latent prints will rarely be as clearly as inked prints, and thus
we added visual noise to the prints to test whether experts could better tolerate the degraded viewing conditions than novices. The
images below demonstrate the visual noise manipulation:
The third factor was a manipulation meant to mimic the fact that latent prints are rarely complete copies of the inked versions.
Usually latent prints are partial prints, and may be patchy due to textures on the contact surface. To produce testing conditions that
were similar to partial prints, we constructed masks that reduced or eliminated portions of the fingerprint feature. An example is
shown below.
Sometimes the compliment masks were used, which allows testing for configural properties as described below.
All three factors were factorially combined, so that the partial image could be shown in noise or free of noise. An example of a
partial print shown in noise is shown below:
There were a total of 8 conditions in experiment, which were factorial combinations of delay (short vs. long), noise (no noise or
noise added) and partial (full image or just partial displayed).
To further mimic conditions present during a fingerprint match process, the features were rotated at test up to 25 degrees and the
brightness was altered. This forces observers to rely on features that survive such transformations and ignore smaller features like a
piece of lint on the inked print which would not be part of a latent print.
Each subject performed 18 replications in each of the 8 conditions for a total of 144 trials.
Results
The results are presented below. Where significant effects or interactions are reported, these are significant at the .05 level in a
repeated measures analysis of variance (ANOVA) with the three within-subject factors (delay, noise and partial) and one betweensubject factor (novice vs. expert). Complete statistical analyses are available upon request from Tom Busey (busey@indiana.edu).
Not surprisingly, fingerprint experts performed much better than novices. Several conditions produced near-ceiling performance,
while some conditions for the novices produced performance at chance. To deal with the potential compression that can occur at the
extremes of the scale, we converted performance to d', which is a measure of the observer's sensitivity in the task i. All differences
reported below are for the d' data, but all effects were consistent in the percent correct data as well at the .05 level.
Experts:
Experts show reduced performance when the feature is presented in noise, as well as when the feature is partially masked.
Somewhat surprisingly, however, is the result that experts are not affected by the delay. Thus they appear to be able to hold features
in memory for longer periods of time.
It is perhaps not surprising that partially masking the feature or presenting it in noise reduces performance. However, the experts
show an interaction between these two factors, such that presented the partially masked image in noise causes a much greater
reduction in performance than would be expected on the basis of the effects of noise and partial masking.
Novices:
Unlike Experts, Novices are affected by the longer delay. Thus they seem to be less able to hold each fingerprint feature in memory
as well as experts can. Novices also show the same main effects of partial masking and noise as experts do, but no interaction
between partial masking and noise. In fact, the novices show no interactions at all.
Comparisons across groups:
The different patterns of results described above leads to a number of interesting interactions with group membership. All three
main variables (delay, noise and partial masking) show interactions with group type, which means that the three manipulations
affect the two groups differently (the partial masking result is only marginally significant). The interaction between partial masking
and added noise was significant for experts but not novices, and this produces an interaction between the two factors and group
membership.
Discussion
What factors underlie the strong performance demonstrated by Experts? While they demonstrate better ability to retain images in
memory over longer delays, it may also be the case that Experts process the fingerprint features in a different way from novices.
One possible strategy adopted by experts might be that of configural or holistic processing.
Holistic processing
The partial masks were carefully constructed so that each one presented only half of the information available in the intact
fingerprint. This was done by sometimes inverting the mask so that its compliment would be used to mask the fingerprint. Given
this design it is possible to predict how well observers will do in the intact condition based on how well they have done on the
partially masked condition. There are some statistical issues to consider that relate to issues like guessing, but basically the logic
boils down to this: in the partially masked case, the observer has only one chance to get enough information from the test images in
order to make the correct decision. In the intact case, however, the observer has two chances, essentially one chance from each of
the two partial images that make up an intact image. Are the experts and novices making use of all the information in the intact
image?
Surprisingly, the answer depends on whether the images are presented in noise or not. Without noise, both novices and experts seem
to be underutilizing the information in the intact image. That is, they do worse on the intact image than we would have expected
given their performance on partial images. For some reason, both novices and experts seem to be underperforming when it comes to
intact images.
There is a difference, however, when the partial and intact images are presented in noise. This condition is particularly interesting,
since latent prints are often very noisy, and while the visual noise we add may not be exactly the same as the noise caused by
environmental factors in a latent print, one might expect that if there is a difference between novices and experts it may be revealed
under high-noise testing conditions. When noise is added to both the intact and partial test prints, novices perform about as well as
expected on the intact images. However, and this is the most interesting finding, experts perform much better on the intact images
than one would predict based on their partial print performance. Somehow for experts, the whole is greater than the sum of the
parts, and this is consistent with some form of holistic processing that may come about when experts talk about 'using the totality of
the image' rather than focusing on individual features. This suggests that features are not processed individually, but affect each
other, and this occurs more often in the intact print.
Summary
In summary, there are a number of factors that differentiate Expert Examiners from Novices. Experts show overall much better
performance than novices. In addition, Experts are unaffected by long delays imposed between the study image and the test image,
while novices show a marked decrease in performance. Both Experts and Novices are affected by adding noise and partially
masking the test images, but Experts show an interaction between the added noise and partial masking procedures. This suggests
that they change how they view images embedded in noise. One possibility is that experts adopt a strategy of doing more holistic
processing when viewing images in noise, which works for intact but not partial images.
i
For conversion to d', perfect scores were converted to a value equal to (1-(1/(2N)) where N is the number of trials in that
condition, usually 18. This is not an ideal solution, but performance for novices was in danger of reaching chance in some
conditions, and so we feel that we chose stimulus durations that were appropriate for both sets of observers. We could reduce the
duration for the experts, but at 200 ms it is already fairly brief, and we run the risk of changing the nature of the information they
acquire by going to shorter displays. Thus at this point we are content to analyze both the d' and percent correct data and if they give
the same result assume that ceiling effects are not a major determinant of our interactions.