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OCR for page 22
Table 43 Results of t-test for comparison of measured and target percent round on #14 sieve of Type 5 samples
Method of No. Average % % Rejection
Measurement-- of Round, Round, Probability
Sample Type Labs Measured Target Sx Computed t Critical t Decision (p)
Roundometer-- 8 83.4 90.0 4.14 4.534 2.365 Reject 0.003
Type 5
COM-A-b/l-- 8 89.2 90.0 1.13 2.049 2.365 Accept 0.080
Type 5
COM-A-SPHT-- 4 92.1 90.0 1.13 3.621 3.182 Reject 0.036
Type 5
COM-B-NSP-- 1 91.0 90.0 -- -- -- -- --
Type 5
5 beads, the COM-A b/l parameter provided a signif- one-sample t-test. Following the ASTM E691 metho-
icantly more accurate roundness measurement than dology, both within- and between-laboratory variabil-
did the mechanical roundometer. However, the SPHT ity of the computerized and mechanical methods was
parameter of COM-A failed to measure the roundness also determined for each sieve size of each glass
of any of the glass bead types correctly. This may bead type.
result from shortcomings of the parameter or a poor The computed t-values and the computed within-
choice of threshold value. and between-laboratory standard deviations for each
glass bead type were compared to determine the
CONCLUSIONS accuracy and precision of the different methods in
measuring the properties of the glass beads. For each
Three methods for size and roundness measure- glass bead type, comparison was made between the
ment were evaluated in this research: two comput- statistics corresponding to the sieve sizes retaining
erized optical methods (COM-A and COM-B) and the highest mass percentage of the beads. This was
the traditional mechanical methods--sieving and done because these sieves provided the most precise
roundometer following ASTM D1214 and ASTM measurements of the glass bead type.
D1155, respectively. The largest number of datasets Because only one laboratory provided size and
were obtained for mechanical sieving and the roundness measurements of Type 3 and 5 glass beads
roundometer: 14 datasets of size distribution and using the COM-B instrument, the discussion of pre-
11 of roundness data. COM-A users provided eight cision and bias for measurements of the properties of
datasets, which included both size and roundness Type 3 and Type 5 glass beads does not apply to the
data. COM-B datasets were received from four lab- COM-B results. The results of the comparison are
oratories for small size glass beads (Type 1) and summarized as follows:
from one laboratory for larger glass beads (Types 3
and 5). The samples upon which these tests were run · Mechanical sieving, the COM-A device, and
were carefully prepared via mechanical sieving and the COM-B instrument were used to measure
mechanical roundness measurement, so accuracy in the size of Type 1 glass beads. Analysis of the
the ILS for the roundometer, COM-A, and COM-B mass percent retained in the largest size class of
results is defined as how close these measurement the Type 1 samples indicated that of the three
methods came to the original mechanical sieving methods of measurement, the COM-B device
and roundness measurements, allowing for the error provided the most accurate measurement of
introduced in the sample preparation process. the size of Type 1 glass beads. With respect
The ILS data received from participating labora- to variability, the mechanical sieve provided the
tories were statistically analyzed for precision. The smallest within-laboratory standard deviation
significance of the bias for each method between for measuring the size of the small beads. How-
measured and target values was evaluated separately ever, the between-laboratory precisions of the
for each sieve size of each glass bead type using the three methods were very similar.
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OCR for page 23
· Data from the mechanical sieve and the COM- Among the three combinations, COM-A-b/l
A device were used to develop precision and provided the most accurate and precise mea-
bias estimates for size measurement of the surement of the roundness of Type 3 and Type
Type 3 and Type 5 beads. Between the two 5 glass beads. The mechanical roundometer did
methods, the COM-A instrument measured the not provide equivalent accuracy and precision
size of the Type 3 and Type 5 glass beads with in measuring the mass percent round of Type 3
more accuracy than the mechanical sieves. In and Type 5 glass beads.
addition, the COM-A device provided smaller Taken together, these results suggest that com-
within- and between-laboratory variability than puterized optical methods are preferable to the tra-
did the mechanical sieves. ditional mechanical methods for measuring the
· Four combinations of methods and parameters size and roundness of glass beads. The improved
were used for measuring the roundness of statistics of the b/l parameter for the larger glass
Type 1 glass beads: mechanical roundometer, beads indicated the advantage of COM-A over
COM-A-b/l, COM-A-SPHT, and COM-B- the roundometer for roundness measurement of
T/L. Among the four combinations, the COM- the larger glass beads. Although a smaller number
B-T/L provided the most accurate measurement of laboratories provided data using the COM-B
of roundness of small glass beads. However, device, both size and roundness of the Type 1 glass
COM-A-b/l provided the most precise within- beads were correctly measured by the COM-B
and between-laboratory measurements. The instrument.
mechanical roundometer did not provide equiv- A draft practice in AASHTO standard format for
alent accuracy and precision for measuring the determining the size and roundness of glass beads uti-
roundness of Type 1 glass beads. lized in traffic markings using the computerized opti-
· Three combinations of methods and parameters cal method is provided in Appendix A. This draft prac-
were used for measuring the roundness of tice includes a precision and bias statement for the
Type 3 and Type 5 glass beads: the mechanical computerized optical method measurements based on
roundometer, COM-A-b/l, and COM-A-SPHT. the results of this research.
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