16 Questions
The mean is the most widely used measure of central value in statistics.
True
The median is the result in the exact middle of replicate data when arranged in order.
True
Analyzing replicates allows us to estimate the precision of the individual measurements.
True
The central value of a set is usually higher than any of the individual results.
False
When calculating the median for an even number of results, the average of the middle pair is used.
True
Replicates are not necessary when trying to improve the reliability of analytical procedures.
False
Precision describes the agreement among several results obtained in the same way.
True
Accuracy measures how scattered data is around the mean value.
False
Systematic errors may cause the mean of a data set to differ from the accepted value.
True
Random errors are also known as determinate errors.
False
Personal errors in measurements often require personal judgments.
True
Systematic method errors are usually related to the incorrect use of measuring devices.
False
Proportional errors are caused by random fluctuations in the measurement process.
False
Constant errors have a fixed magnitude regardless of the quantity being measured.
True
Indeterminate errors cause data to be scattered around a mean value.
True
Precision is expressed in terms of either absolute or relative error.
False
Study Notes
Improving Reliability
- To improve the reliability of results, 2-5 portions (replicates) of a sample are carried through an entire analytical procedure.
- The central value of a set of measurements is considered the "best" estimate, often represented by the mean or median.
The Mean and the Median
- The mean (arithmetic mean or average) is the most widely used measure of central value, calculated by dividing the sum of replicate measurements by the number of measurements.
- The median is the middle result when replicate data are arranged in increasing or decreasing order.
- For an odd number of results, the median is the middle result; for an even number, the average value of the middle pair is used.
Precision and Accuracy
- Precision describes the reproducibility of measurements, or the closeness of results obtained in exactly the same way.
- Accuracy indicates the closeness of the measurement to the true or accepted value, expressed by the error.
- Precision can be determined by measuring replicate samples, while accuracy is often more difficult to determine due to unknown true values.
Types of Errors
- Random (indeterminate) error causes data to scatter symmetrically around a mean value.
- Systematic (determinate) error causes the mean of a data set to differ from the accepted value.
- Gross error is a rare, large error that can cause a result to be either high or low.
Systematic Errors
- Instrumental errors occur due to measuring devices, such as pipets, burets, and volumetric flasks, holding or delivering volumes slightly different from those indicated.
- Method errors occur due to non-ideal chemical or physical behavior of reagents and reactions.
- Personal errors occur due to personal judgments, such as estimating the position of a pointer between two scale divisions.
Effect of Systematic Errors
- Constant errors become more serious as the size of the quantity measured decreases.
- Proportional errors occur due to the presence of interfering contaminants in the sample, causing high results for the percentage of copper.
Learn about the important terms precision and accuracy in analytical chemistry. Understand how replicates are used to improve reliability and obtain information about result variability. Discover the reasons behind analyzing replicates and the concept of central value in a set of measurements.
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