1 Establishing a standard work sampling method for mastication ...

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Figure 1. This cost tree diagram illustrates the process whereby collected data are calculated to determine the mastication cost. Results and Discussion Evaluation of three work sampling methods The mastication operation yielded 16 hours of machine operation, resulting in 23,325 observations for the benchmark and 2,111 observations for each of the three work sampling methods that were evaluated. There was strong evidence that none of the work sampling delay ratios were significantly different from the benchmark (p >0.01667; Table 3). 6
Table 3. The results for the three separate 2-proportion tests for difference show that none of the work sampling methods produced a delay ratio that was significantly different from the benchmark. Sampling method benchmark Delay ratio (proportion) 0.113 Estimate for difference from benchmark 98.33% CI 1 for difference from benchmark p-value Different from benchmark 2 SyRS 0.114 -0.001 (-0.018, 0.016) 0.873 No 3 SRS 0.116 -0.003 (-0.021, 0.014) 0.625 No 4 StNCRS 0.123 -0.010 (-0.027, 0.008) 0.180 No 1 Confidence Interval, 98.33% because alpha level set at 0.01667 for 95% family-wise confidence in all three conclusions simultaneously (α = 0.05/3 = 0.01667) 2 Systematic Random Sampling 3 Simple Random Sampling 4 Stratified Non-Continuous Sampling As there was no significant difference in the accuracy of the sampling methods, the strengths and limitations of each were evaluated to establish which is the most easily implemented. Drawing sample times was the easiest using SyRS, followed by SRS and lastly StNCRS. Our conclusion was also supported by the findings from Pape (1992). Work sampling can be mentally fatiguing (Olsen and Kellogg 1983), so it is important to select a method where collecting observations is easy. In this regard, the preferred method was SyRS, followed by StNCRS and lastly SRS (Pape 1992). SRS was judged to be the most difficult in this category because by random chance there might be a cluster of many sampling times; this may result in insufficient time between observations to realize what the machine is doing before the subsequent observation (Miyata et al. 1981; Olsen and Kellogg 1983; Pape 1992). By combining the outcomes of these ease of implementation measures SyRS was shown to be preferable, as was also found by Gardner and Schillings (1969). Despite this conclusion, there are dangers to SyRS. The primary caution to work sampling is that important events between observations can be missed, especially as the interval between observations becomes longer. This may be overcome with short intervals such as Bolding’s (2006) use of a 20-sec. interval or the 30-sec. interval recommended by Olsen and Kellogg (1983). However, deviating from the calculated interval length may result in unequal sampling; this may result in bias (Pape 1992). Using SRS, the sixteen hours of video were used to analyze the productivity of the masticator. The masticator’s operation yielded 11.3% delay, 85.7% productive, and 3% research delay. The 7
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