This study aimed to assess the inter- and intra-examiner reliability, the technical error of measurement and the validity of instruments for measuring weight, height and waist circumference.Ī convenience sample of 130 adults working in a selected office setting was chosen to participate in the study, subject to the inclusion and exclusion study criteria. Making many measurements will also reduce the total error proportionally to the square root of the number of measurements taken.The Third National Health and Morbidity Survey Malaysia 2006 includes a nutritional status assessment of children. For serious work, like publications, you should perform many sample measurements and calculate the error statistically. On the other hand, overly ambitious errors will likely give a result that is overly precise but inaccurate when the experiment is duplicated by others. If you use a high or conservative measuring error then you will get an unnecessarily imprecise result.
But you have to make this judgement call based on the readability of the setup. Thus, 96% of guesses for sure would be in the interval 9.3cm to 9.7cm and 68% of the guesses would realistically be between 9.4cm and 9.6cm. The reading error of 0.1cm is because we can intuitively picture that the largest guess one might give is 9.7cm and lowest would be 9.3cm. Really, the measurements should add in quadrature as SQRT((0.1cm^2) + (0.1cm^2)) = +/- 0.14cm. Both measurements of length do contribute to the error so we add the errors but this is actually an approximation being generously conservative. Thus, the total length is 9.6 +/- 0.2 cm. In your example it looks like the 2 ends are -0.1cm and 9.5cm with errors of +-0.1cm. In your example, the smallest increments are 1 cm, so this ruler should easily give a measurements error of +/- 0.1cm. That is, no parallax error and the ruler is close enough to the device being measured to guess at 1/10 increments of a mm. The reading error for a standard ruler with mm increments is +/- 0.1mm under perfect conditions. This is often answered incorrectly in Google searches on measurements. Physicists use the largest error, which in this case is the reading error. Related question/answers with reference to combining errorsįirst, the accuracy of the ruler because of manufacturing errors is generally smaller than the reading error of the ruler. Which is approximately correct, but a bit pessimistic. If you had to measure two positions to calculate a length then you might haveĪnd from that we can make an estimate of error in $X$ with For example, if you could measure something 10 times and you get slightly different values each time then the mean is your best value for the measurement and the standard deviation divided by the square root of the number of measurements is the uncertainty or error in the measurement. Ideally it would be good to have an objective way to measure error. The problem with estimation is that it is subjective. Often when measuring length with a ruler we have to estimate what the length is and judge how accurately we can make the measurement. 6, then you might estimate that the measurement was say $6.0 \pm 0.1 cm$.
If you are measuring in a laboratory with a ruler like the one in your diagram then I would say for a length of $9.5 cm$ you would be able to see with your eye that the length is say $9.5 \pm 0.2 cm$ and if it actually was on one of the markings, e.g.
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