5: Lab Technician's Guide to Accuracy, Precision, and Reliability

Lab Technician's Guide to Accuracy, Precision, and Reliability

The world of laboratory science revolves around measurements, guided by essential concepts like standards, calibration, and traceability. These principles form the foundation for ensuring reliable and consistent measurements, impacting the precision and accuracy of data generated in laboratories. Let's explore these ideas to understand their importance in the realm of scientific measurements.

Metrology, which is the science and practice of measurements, plays a crucial role in achieving consistency in measurement practices on a global scale. At its core, metrology aims for international uniformity, and this objective is achieved through collaborative efforts. These efforts have resulted in the establishment of the SI (Le Système International d'Unités) measurement system. The SI system serves as a universal reference, providing authoritative definitions for various units of measurement and creating a shared language for scientists and technicians around the world.

At the heart of ensuring trustworthy measurements is the idea of standards. Essentially, a standard is a physical representation of a unit, providing a tangible reference point for measurements. Take, for instance, the kilogram standard, traditionally represented by a platinum-iridium bar. However, this physical representation faced challenges, leading to a significant change in 2019. The definition of the kilogram shifted to rely on Planck's constant, marking a move toward a more stable and secure foundation for this fundamental unit of mass.

Calibration, a key process for maintaining measurement accuracy, involves adjusting measuring instruments to ensure they provide accurate readings. This adjustment is carried out using external standards to align the instrument's readings with accepted values. Calibration comes in various forms, ranging from adjusting an instrument to match a standard's value to evaluating the performance of a measuring system. For instance, a laboratory pH meter may need periodic recalibration to correct for reading drift caused by factors like aging or environmental changes. The process entails adjusting the pH meter's readings based on the pH values of standard solutions with known pH levels.

However, calibration introduces a crucial concept: tolerance. Tolerance represents the acceptable amount of error in the calibration of a specific item, recognizing that completely eliminating error is not achievable. The challenge lies in defining an acceptable range of deviation. For instance, a "100 g" Class 1 mass standard might have a tolerance of 1.2 mg, indicating that its true mass should fall within the range of 100.0012 to 99.9988 g. In a broader sense, calibration serves as a formal assessment of a measuring instrument, ensuring its readings align with standard values.

In addition to calibration, the concept of verification serves as a complementary practice to ensure the proper functioning of instruments. Verification entails regular checks on instrument performance, contrasting with the more rigorous adjustment process involved in calibration. Unlike calibration, verification is a simpler evaluation conducted in the user's laboratory, documented to keep track of the instrument's functionality.

Moving on to traceability, a crucial metrological concept emphasizes the necessity of linking measurements to an unbroken chain of comparisons to guarantee reliability. The roots of traceability can be traced back to ancient civilizations, exemplified by the ancient Egyptians using the pharaoh's arm as the national standard of length (Figure 5.1). However, given the impracticality of having the pharaoh present for all measurements, a reproduction of his arm's length using a granite rod was created. This rod, in turn, served as a reference for creating wooden measuring sticks used by workers in various applications, establishing traceability to the original standard.

What is Traceability?
    Imagine you're baking cookies 🍪, and the recipe says to add 1 cup of flour. How do you know your cup is the right size? You probably use a measuring cup, but how do we know that cup is accurate? That’s where traceability comes in! Traceability means being able to track and verify measurements by comparing them to a reliable standard. This ensures that measurements are consistent and accurate across different times and places. But where did this idea come from? 🤔 Let’s travel back in time to ancient civilizations to find out!

Ancient Egypt: The Pharaoh’s Arm as a Standard
    Thousands of years ago, the Ancient Egyptians (around 3000 BCE) needed a way to measure lengths accurately. They built massive pyramids, temples, and canals, so having precise measurements was crucial. How Did They Measure Things? Instead of rulers and measuring tapes, the Egyptians used the cubit, a unit of length based on the pharaoh’s arm. Why Was This Important?  It ensured all buildings, walls, and tools were the same size. It prevented disputes over measurements—everyone had to follow the same standard. It was an early system of traceability, making sure measurements stayed consistent across Egypt.

• The Royal Cubit = the length from the pharaoh’s elbow to the tip of his fingers 
• Craftsmen and builders had to use a stone or wooden cubit rod based on the pharaoh’s arm to ensure accuracy.
• The official standard cubit was kept in temples, and workers had to compare their measuring rods to it.

Other Ancient Measurement Systems
    The Egyptians weren’t the only ones using body parts to measure things! Other civilizations had their own measurement systems:

• Ancient Mesopotamia (Sumerians & Babylonians)
  • Used the finger, hand, and foot as basic units.
  • Created standardized weights for trade using stones and metals.
•  Ancient Greece & Rome
  • The Roman foot (pes) became a widely used length standard.
  • They introduced the mile (from the Latin "mille passus" = 1,000 steps).
•  Ancient China
  • Developed their own system based on grains of rice for weight.
  • Used body parts like the thumb, handspan, and foot for length.

Traceability in Modern Times
    Today, we don’t use pharaohs’ arms to measure things anymore! Instead, we have international measurement standards, such as:

• Meters & Kilograms – Defined by precise scientific methods.
• Atomic Clocks – Used to measure time with incredible accuracy.
• Standardized Units – Used in medicine, engineering, and science to ensure everything is measured the same way worldwide.

The importance of traceability becomes even more apparent when considering mass standards (Figure 5.1). A mass standard in an individual laboratory is calibrated based on a standard that, in turn, undergoes calibration by comparison with a standard at a national standards laboratory, such as NIST (National Institute of Standards and Technology). This chain of traceability ensures that calibrations and measurements made using that standard are reliable and trustworthy. Manufacturers often emphasize the traceability of their standards in catalogs, underscoring adherence to specific standards during the manufacturing process. In essence, traceability offers a systematic way to demonstrate the reliability of measurements.

Understanding the significance of traceability dispels the misconception that the United States does not use the metric system. While everyday measurements in the U.S. may be expressed in units like miles, inches, and Fahrenheit, there is an underlying connection to the metric system. The unbroken chain of traceable measurements ensures that U.S. measurement units, despite their apparent non-metric nature, are ultimately defined in terms of the SI system.

As we explore the realm of measurements, one fundamental truth emerges—variability is a natural aspect of all observations in nature, measurements included. Imagine the scenario of weighing the same standard ten times using a high-quality balance. Despite sticking to the same steps under consistent conditions, the results would show slight variations. This inherent variability in measurements is what statisticians refer to as random errors, introducing fluctuations where values can be too high or sometimes too low. Simultaneously, systematic errors, arising from sources like instrument malfunctions, contaminated solutions, or environmental inconsistencies, present a subtler challenge. These errors introduce bias, causing measurements to consistently deviate either too high or too low.

In the realm of reliable measurements, two crucial concepts take center stage: accuracy and precision. Despite their common interchangeability in everyday language, these terms hold distinct meanings within the scientific domain.

Precision, in essence, assesses the consistency among a series of measurements or tests. It gauges how closely repeated measurements align with each other, indicating minimal variability under similar conditions. On the flip side, accuracy measures how closely a measurement value aligns with the true or accepted value. Accurate measurements not only show consistency but also closely match the accepted reference values. It's worth noting that a series of measurements can be precise but not accurate, or accurate but not precise. A "good" measurement, however, achieves both accuracy and precision. For instance, if measurements of a standard consistently yield the same value but deviate from the accepted reference value, they are precise but not accurate. Conversely, if measurements cluster closely around the accepted reference value but display significant variability, they are accurate but not precise. See Figure 5.2 for example.

The pursuit of a perfect measurement every time involves delicately navigating the balance between accuracy and precision. Although achieving both may pose challenges, laboratories aim to optimize their measurement processes. This optimization entails minimizing systematic errors, addressing sources of variability, and implementing robust quality control measures.

Lab technicians strive for precise measurements despite challenges like natural variability, instrument constraints, and unpredictable errors. They minimize errors by strictly following protocols, regularly calibrating instruments, and pinpointing sources of systematic errors. Dedication to quality control elevates measurement precision and accuracy. By acknowledging uncertainty and diligently reducing errors, they actively contribute to the creation of reliable and trustworthy data.

In conclusion, aiming for perfect measurements is a commendable goal, though it comes with challenges. The dynamics of accuracy, precision, and uncertainty, combined with the impact of random errors, make achieving perfection unrealistic. Nevertheless, with careful attention to detail, dedication to quality practices, and a realistic approach to uncertainty, you can improve the reliability and trustworthiness of your measurements.

Chapter tile image from: 2.2: PART I- Metrics is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Donna Barron.