Metrology Adapted from Introduction to Metrology from the Madison Area Technical College, Biotechnology Project (Lisa Seidman)

What is metrology? The study of measurements Measurements are quantitative observations; numerical descriptions Measurements are part of the daily routine in a biotech lab Measurements are expected to be “good”

What is a “good” measurement? If you weigh at home and then at the doctor’s office and get a different weight, which is correct? Did your weight change (sample issue)? Is one or both scales wrong (instrument issue)? How do you know which of these is correct?

What is a “good” measurement? A “good” measurement is one that can be trusted when making decisions Decisions are made daily on whether measurements are good enough, but they are made subconsciously and often by different people Decisions need to be conscious and consistent.

Metrology Vocabulary Unit of measurement Accuracy Precision Standards Calibration Verification Traceability Tolerance Errors Uncertainty

Question Would you rather me give you the one worth one or worth five?

Units Units define measurements Units give the numbers value Definition set by international SI system

Accuracy vs Precision Accuracy is how close an individual value is to the true or accepted value Precision is the consistency of a series of measurements From Basic Laboratory Methods for Biotechnology: Textbook and Laboratory Reference, Seidman and Moore, 2000

Measurements can be: Accurate and precise (best) Accurate and imprecise (user error) Inaccurate but precise (instrument error) Inaccurate and imprecise

Expressions Accuracy % error = True value – measured value X 100% True value Precision Expression of variability Take the mean (average) Calculate how much each measurement deviates from mean Take an average of the deviation, so it is the average deviation from the mean

Recording measured values Record measured values (or large counts) with correct number of significant figures Don’t add extra zeros; don’t drop ones that are significant With digital reading, record exactly what it says; assume the last value is estimated With analog values, record all measured values plus one that is estimated

Significant Figures The digits always count. (51 has 2) Zeroes between the digits always count. (501 has 3) Zeroes in the beginning of a number never count. ( only has 3) Zeroes at the end of a number count only if there is a written decimal point. (5010 has 3, has 4)

Rounding Greater than or equal to 5 then round up Less than 5 then round down When adding or subtracting, the number of decimal places in the result equals the smallest number of decimal places in the input numbers. When multiplying or dividing, the number of significant figures in the result equals the smallest number of significant figures in the input numbers.

Scientific Notation The coefficient must be greater than or equal to 1 and less than 10. The base must be 10. The exponent must show the number of decimal places that the decimal needs to be moved to change the number to standard notation. A negative exponent means that the decimal is moved to the left when changing to standard notation.

Standards Measurements made in accordance with an external authority A standard is an external authority They are physical objects, the properties of which are known with sufficient accuracy to be used to evaluate other items Units are unaffected by the environment, but standards are Also solutions or documents

Calibration Bringing a measuring system into accordance with external authority, using standards For example, calibrating a balance Use standards that have known masses Relate response of your balance to units of kg

Verification Check of the performance of an instrument or method without adjusting it.

Tolerance Amount of error that is allowed in the calibration of a particular item. National and international standards specify tolerances.

Example Standards for balance calibration can have slight variation from “true” value Highest quality 100 g standards have a tolerance of mg g Leads to uncertainty in all weight measurements

Traceability The chain of calibrations, genealogy, that establishes the value of a standard or measurement In the U.S. traceability for most physical and some chemical standards goes back to NIST(National Institute of Standards and Technology)

Error Error is responsible for the difference between a measured value and the “true” value Three types of error: Gross (blunders) Random Systematic

Random Erros Random errors are errors that cannot be eliminated. They are variability and no one knows why. Maybe humidity, pressure, etc. This is why we take several measurements and average them to get best estimate of true value Random error leads to loss of precision

Systemic Error Defined as measurements that are consistently too high or too low, bias Many causes, contaminated solutions, malfunctioning instruments, temperature fluctuations, etc., etc. Technician controls sources of systematic error and should try to eliminate them, if possible Impacts accuracy so try not to repeat them

Uncertainty Estimate of the inaccuracy of a measurement that includes both the random and systematic components. Errors lead to uncertainty in measurements Can never know the exact, “true” value for any measurement. Idea of a “true” value is abstract – never knowable. In practice, get close enough

Which ruler gives the length of the arrow with the most certainty?