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To estimate the accuracy of the Dektak's measurements we have to combine the error of the calibration with the error from the limit on the resolution and the scatter of repeated measurements. This is shown graphically on the right. You can see an uncertainty budget for the Dektak measurements here (made by Rebecca Ettlinger in 2020): [[Media:uncertainty budget DektakXT Oct2020.xlsx]]. It is based on the assumption that all the error sources are independent and can therefore be added by the sum of squares method. Or as expressed by the international bureau of standards and measures, the "combined standard uncertainty [..] is the positive square root of the combined variances[..]". See this document: [[Media:JCGM_100_2008_E.pdf]].  
To estimate the accuracy of the Dektak's measurements we have to combine the error of the calibration with the error from the limit on the resolution and the scatter of repeated measurements. This is shown graphically on the right. You can see an uncertainty budget for the Dektak measurements here (made by Rebecca Ettlinger in 2020): [[Media:uncertainty budget DektakXT Oct2020.xlsx]]. It is based on the assumption that all the error sources are independent and can therefore be added by the sum of squares method. Or as expressed by the international bureau of standards and measures, the "combined standard uncertainty [..] is the positive square root of the combined variances[..]". See this document: [[Media:JCGM_100_2008_E.pdf]].  


The error stemming from the uncertainty on the calibration standard dominates for all ranges except the 1 mm range, where the resolution also plays a role. This leads to the 95 % confidence intervals listed above in the table: just over 18 nm for the smallest range, just over 0.2 µm for the medium ranges, and about 0.6 µm for the 1 mm range.  
The error stemming from the uncertainty on the calibration standard dominates for the 6.5 micron range, while for the other ranges the scatter of repeated measurements is also important. Using the sum of squares method leads to the 95 % confidence intervals listed above in the table: just over 18 nm for the smallest range and around 0.2 µm for the other ranges.  


As noted above, be aware that if you have a step height that is difficult to measure, the scatter of repeated measurements could easily lead to larger confidence intervals. To improve the accuracy of your particular measurement, you should repeat the measurement several times and estimate the standard deviation to verify that the uncertainty on the height of the calibration standard dominates over the uncertainty of the scatter of your measurements. If the scatter of your measurements is large, you can use our uncertainty budget for calculating the cumulative uncertainty for your own sample.
As noted above, be aware that if you have a step height that is difficult to measure, the scatter of repeated measurements could easily lead to larger confidence intervals. To improve the accuracy of your particular measurement, you should repeat the measurement several times and estimate the standard deviation. If the scatter of your measurements is large, you can use our uncertainty budget to calculate the cumulative uncertainty for your own sample.


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