Kathleen Z. Rooney
A mass grave is exhumed in Argentina by a team of forensic anthropologists. These scientists sift through bones for the identities of the desaparecidos, ""the disappeared ones.""
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This horrific scene has been played out again and again, from Serbia, to Rwanda, to the site of the fallen Twin Towers. Forensic techniques have identified bodies to bring closure to bereaved families, and to be used as criminal evidence at trial. How are the age, gender and appearance of a person revealed by the shape and size of their bones? Do bones have important differences in size, appearance and composition? What can we learn about how to narrow our search for answers using statistical measures?
Types of information from bones
Bones can reveal much about a victim. Location of discovery, level of decay, size and shape are all useful indicators. Finding an intact skeleton, one can use the height to predict the age or gender using tables of normal curves. This particular concept requires little or no background knowledge and there is excellent data available through the United States Government''s Centers for Disease Control (CDC) site in The National Health and Nutrition Examination Survey (NHANES)
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. An example introducing the use of the normal distribution in calculating percentiles will be included in the lessons for this unit.
In order to carefully determine the age of a victim, an X-–ray of the bones may be helpful. As a person grows, their long bones grow outward at the ends, called the epiphysis. Before the bones are fully developed, there is an area that is more cartilage than bone. The size of that area shrinks as the person ages. An X-–ray can be useful to measure the size of the area known as the epiphyseal plate. If the person is fully grown, the plate is simply a line. The measure of the epiphyseal plate can be compared to sample distributions in order to estimate the age of the skeleton.
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Additionally, the numbers of permanent teeth in a skull are an indication of the age of the individual.
A single bone can be used to predict the height of an individual, again using sample data. Students can practice this by creating a sample using the NHANES data. Students can use linear regression techniques to predict height from the length of a single bone, see the lessons area for this example. Regression can also be used to predict weight based on skeletal evidence. Using X-–rays of the femur, the weight of an individual is correlated with the diameter of the femur.
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Perhaps we have found only a piece of the person''s pelvis. Can we determine gender? We have seen that predicting gender using height alone can be inaccurate. There is a generous overlap between height that is predictably male and that is predictably female. In the lessons, we will look at gender differences in bones. Students will look at an image of the pelvis and identify key parts of the bones.
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Using another distribution, students will predict the person''s gender based on the associated probabilities in the study.
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The skeleton is a long lasting piece of our equipment. Using the bones to estimate time of death is not a very useful indicator. Once the soft tissue of our body has decayed, in a time span between two and eight months, our bones will remain fairly intact for a long period of time. Carbon dating can be used, or age can be estimated by using historical artifacts found at the site.