This target has been struck with a high degree of precision, yet a low degree of accuracy.
This target has been hit with a high degree of accuracy, yet a low degree of precision.
Accuracy
Accuracy refers to the agreement between experimental data and a known value. You can think of it in terms of a bullseye in which the target is hit close to the center, yet the marks in the target aren't necessarily close to each other.
Accuracy refers to the agreement between experimental data and a known value. You can think of it in terms of a bullseye in which the target is hit close to the center, yet the marks in the target aren't necessarily close to each other.
The "trueness" or the closeness of the analytical result to the "true" value. It is constituted by a combination of random and systematic errors (precision and bias) and cannot be quantified directly. The test result may be a mean of several values. An accurate determination produces a "true" quantitative value, i.e. it is precise and free of bias.
Precision
Precision refers to how well experimental values agree with each other. If you hit a bullseye precisely, then you are able to hit the same spot on the target each time, even though that spot may be distant from the center.
The closeness with which results of replicate analyses of a sample agree. It is a measure of dispersion or scattering around the mean value and usually expressed in terms of standard deviation, standard error or a range (difference between the highest and the lowest result).
Keep in Mind - Data can be very precise such that each data point is close to the others, yet contain a high degree of experimental error.
Bias
The consistent deviation of analytical results from the "true" value caused by systematic errors in a procedure. Bias is the opposite but most used measure for "trueness" which is the agreement of the mean of analytical results with the true value, i.e. excluding the contribution of randomness represented in precision. There are several components contributing to bias:
The consistent deviation of analytical results from the "true" value caused by systematic errors in a procedure. Bias is the opposite but most used measure for "trueness" which is the agreement of the mean of analytical results with the true value, i.e. excluding the contribution of randomness represented in precision. There are several components contributing to bias:
1. Method bias
The difference between the (mean) test result obtained from a number of laboratories using the same method and an accepted reference value. The method bias may depend on the analyte level.
The difference between the (mean) test result obtained from a number of laboratories using the same method and an accepted reference value. The method bias may depend on the analyte level.
2. Laboratory bias
The difference between the (mean) test result from a particular laboratory and the accepted reference value.
The difference between the (mean) test result from a particular laboratory and the accepted reference value.
3. Sample bias
The difference between the mean of replicate test results of a sample and the ("true") value of the target population from which the sample was taken. In practice, for a laboratory this refers mainly to sample preparation, subsampling and weighing techniques. Whether a sample is representative for the population in the field is an extremely important aspect but usually falls outside the responsibility of the laboratory (in some cases laboratories have their own field sampling personnel).
The difference between the mean of replicate test results of a sample and the ("true") value of the target population from which the sample was taken. In practice, for a laboratory this refers mainly to sample preparation, subsampling and weighing techniques. Whether a sample is representative for the population in the field is an extremely important aspect but usually falls outside the responsibility of the laboratory (in some cases laboratories have their own field sampling personnel).
nice topic related to daily analysis,with nice photo.thanks for adding photograph, most awaiting one.
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