Applying the Scientific Method

different aspects: interior ballistics (bullet behavior within the confines of the barrel), exterior ballistics (bullet behavior upon exiting the barrel), and terminal ballistics (bullet behavior upon impacting a target). Wound ballistics is a specialized area of terminal ballistics relating to the behavior of bullets striking human or animal targets.

Tool mark examination, on the other hand, is limited to the determination of whether a tool mark was made by a particular tool. Tools commonly examined and compared to questioned (evidence) tool marks include pliers, saws, screw drivers, pry bars, hammers, chisels, and the like.

Tool marks are of two different types: impressed and striated. Impressed tool marks result when a tool leaves an impression, or dent, on another surface. An example would be the result of a blow from a hammer on soft wood. Striated tool marks are the result of a combination of force and motion. The example of a gun barrel producing a tool mark on a bullet illustrates the production of striated tool marks. A cut in metal with a pair of shears is another example. The striations that are produced are often visible only under magnification (with a microscope).

The basis for determining that a particular tool produced a certain tool mark is the presence of random, unique marks that only that tool could have left. In order to be able to properly evaluate firearm and tool mark evidence, an examiner must have extensive training and knowledge about manufacturing techniques of firearms and tools. This is required so that the examiner can distinguish between what are random, unique marks, or "individual characteristics," and what are merely "class characteristics" or "subclass characteristics."

Class characteristics are those characteristics exhibited by an entire group or class of tools. An example would be the width of the blade of a particular brand of screwdriver or the cross-sectional diameter of a gun barrel (caliber) of a certain brand and model firearm.

Subclass characteristics are produced incidental to manufacture and can change over time. Thus, only certain members (a subset) of a class or group will exhibit them. An example would be tools made using a mold or die having some defect that carries over to the tool. Once the die or mold wears such that the defect disappears, or the die or mold is replaced, the subclass characteristic does not appear on subsequently manufactured tools. A group of firearms or ammunition components can also exhibit various properties that are limited to that group and not the entire population at large.

Individual characteristics are unique to a particular tool or firearm to the exclusion of all similar tools or firearms. They are accidental or random characteristics that provide a basis for individualization. Individual characteristic may be produced in manufacture and/or through use (wear). Thus, the interior surfaces of gun barrels exhibit unique characteristics due to manufacture and wear.

The actual comparison and ultimate identification of individual characteristics evaluates quantitative and qualitative aspects of the surface contours. Identification is the result of the examiner determining that "sufficient agreement" exists in the surface contours of a test tool mark and an evidence tool mark.

From a practical standpoint it would not be expected that two tool marks would ever be identical. In their article "Criteria for Identification of Tool Marks," which appeared in AFTE Journal in 1998, Jerry Miller and Michael McLean put it like this: "Nothing duplicated in nature has been reportedly found to be exactly the same, and man has not been able to produce things exactly alike."

The Association of Firearm and Tool Mark Examiners allows for "opinions of common origin to be made when the unique surface contours of two tool marks are in sufficient agreement." The definition of sufficient agreement varies somewhat depending on who is asked to define it, but it generally involves the examiner having looked at a large number of tool marks, particularly known nonmatches (tool marks known to have been made by different tools).

Studies of the number of consecutive matching striae have been conducted. Former firearm examiners from California Al Biasotti and John Murdock have proposed a "conservative criteria for identification" as follows:

1. In three-dimensional tool marks identification is possible when at least three each or one group of six consecutive matching striae appear in the same relative position in an evidence tool mark compared with a test tool mark.

2. In two-dimensional tool marks identification is possible when at least two groups of at least five each or one group of eight consecutive matching striae are in agreement.

There is ongoing study and debate on this subject. The lack of universally accepted criteria for identification has fostered legal challenges to tool mark identification. The problem of articulating criteria for identification lies in the fact that comparisons must take into account not only the quantitative aspect (number of consecutive matching striae) but also the quality of the match. Exactly how to define quality of match is somewhat more difficult than establishing the minimum number of consecutive striae for identification.

In the author's opinion the proof of the scientific validity of tool mark evidence lies in the results of studies of consecutively manufactured tools or firearms. In 1994 the author participated in such a test involving consecutively manufactured pistol barrels. This is because carryover tool marks are not likely to occur in the manufacturing process where the same machine, operator, tool, and/or stock are used to produce items one after another.

Testers obtained a group of consecutively manufactured pistol barrels. The purpose of the test was to see if bullets fired through the barrels could be distinguished from one another and, more important, that the bullets could be identified to the specific barrel that fired them. The bottom line was that the bullets were both distinguishable from one another and identifiable to the barrel that fired them.

After decades of court acceptance the scientific validity of tool mark examination has been challenged since the Daubert v. Merrell Dow Pharmaceuticals decision of 1993, based on the lack of clearly articulated criteria for identification. Various studies have been undertaken in an effort to resolve this apparent shortcoming. Further discussion of criteria for identification appears in a following section.

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    What is a science method with diatals ?
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    How does the scientic method apply to forensic science?
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