Full set of Notes with Fill-Ins - San Jose State University

Chem 155 Unit 1 Page 14 of 316Vocabulary: Basic Statistics ReviewPrecisionRandom ErrorAccuracySystematic Error(bias)HistogramProbabilityDistributionIf repeated measurements of the same thing areall very close to one another, then ameasurement is precise. Note that precisemeasurements may not be accurate (see below).Random error is a measure of precision.Random differences between sequentialmeasurements reflect the random error.If a measurement of something is correct, i.e.close to the true value, then that measurement isaccurate.Systematic error is the difference between themean of a population of measurements and thetrue value.A graph of the number or frequency ofoccurences of a certain measurement versus themeasurement value.A theoretical curve of the probability of a certainmeasured value occurring versus the measuredvalue.A histogram of a set of data will often look like a Gaussian probability distribution.AverageThe sum of the measured .xvalues divided by the numbernnof measurements.MedianVariance (σ 2 )Standard Deviation(σ)Relative StandardDeviationPropagaion ofErrorx MEANHalf of the measurements fall above the medianvalue, and half fall below.A measurement ofprecision. The sum of thesquares of the randommeasurement errors.n 1A widely accepted‘standard’ measurement ofprecision. The square root nσof the variance.n 1The standard deviation divided by the mean, andoften expressed as : %RSD=σ/x MEAN 100%.σ 2 n x MEAN x n2When the mean of a set of measurement (x) hasa random error (σ x ), it is reported as x±σ x . If wewish to report the result of a calculation y=f(x)based on x, we propagate the error through thecalculation using a mathematical method.nx MEAN x n2Page 14 of 316