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-the error standard deviation remains fairly constant. The large mean error at the 150 screen location can be attributed to the nonlinearity of the galvanometer characteristic at its range extremes. In the -450 translation line case, the results are skewed with the minimum error mean and the maximum error standard deviation at the -10' screen location. With increasing screen angles, the error mean increases while the error standard deviation decreases and then levels off at a value comparable to that found in the 00 translation line case. The results suggest that change in the lateral position of the headpiece causes skewing. To test this hypothesis, these results were normalized about the lateral position of the headpiece. For each location along the -45* headpiece translation line, the line that best fits the error data across screen angles (excluding the 150 screen angle where the galvanometer nonlinearity is very apparent) was found using linear regression analysis. Using the mean slope of these-lines (the mean slope is 0.026 with a standard deviation of 0.007), the error datum at each headpiece position was adjusted by an amount that would result from laterally shifting a line with this slope an amount equal to that headpiece position's lateral component, after conversion to its equivalent screen angle (i.e., the screen angle when a 0' oriented headpiece is at that lateral position) The means and standard deviations of the adjusted (normalized) data, also presented in Table 4, tend to confirm the stated
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