Measurements and Errors Notes

Examples parallax error reading a buret or could be

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Examples : "Parallax error reading a buret" or could be instrumental noise such as electrical voltage noise of recorder, detector, etc. In order to evaluate data, measurements of experiments are done in replication of samples. In addition, the analyst must try to make the data
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results meaningful. He or she must decide the quality of the results and the limits of error that are acceptable. Accuracy and Precision Accuracy refers to how close a measured value is to the "true" value . Precision is a measure of the reproducibility of a result. It typically is expressed as either a range or the standard deviation, or variance. Note the smaller the standard deviation for example better precision. Uncertainty in Measurements Absolute - the margin of uncertainty of a measurement. ± 0.02 mL or ± 0.001 g Relative - ratio of the absolute uncertainty with respect to the measurement. 18.321 ± 0.001g -->( 0.001g )/(18.321g) = 5 x 10 -5 = Relative Error Percent Relative = 100 x Relative; so Percent Relative = 0.005% Uncertainty and error are sometimes used interchangeably, however uncertainty results from all error sources (both determinate and indeterminate). Absolute and Relative Uncertainty Absolute uncertainty is an expression of the margin of uncertainty associated with a measurement. Absolute uncertainty has the same units as the measurement. Depending on the parameters of measurement usually expressed as the Std. Dev. of the mean for the trials. We tend to correct the systematic errors once they are discovered.
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Relative uncertainty is an expression comparing the size of the absolute uncertainty to the size of its associated measurement. It is a dimensionless quotient. We sometimes use absolute error as a description; absolute error = mean() - true( μ ) Propagation of Error in Calculations How do we propagate error in our calculations? Addition and Subtraction We sum up the squares of the absolute errors and take the square root. That is the answer!!!!
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  • Fall '11
  • Tarr
  • pH, Observational error, absolute uncertainty, relative uncertainty, antilog

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