Life Science

One way to illustrate the difference between "accuracy" and "precision" is by imagining a rifle has been fired at a target. The "bull's eye" at the center of the target represents the desired outcome. Both the locations and the pattern of bullet holes are significant. In Figure 1, the holes are close together, indicating high precision independent of where the average of the holes actually is. However, they are distant from the desired value, the center of the target, so they show low accuracy. In Figure 2, the average is located away from the desired value, the center of the target. This is low accuracy. In addition, the holes are located far apart from each other. This is low precision. In Figure 3, the holes cover a larger area, but the average value is close to the center of the target. This is low precision but high accuracy. In Figure 4, the holes are closely spaced (good precision) and their average is at the center of the target (good accuracy). This is the ideal condition. Illustrating the Terms Figure 4. High precision High accuracy Figure 1. High precision Low accuracy Figure 3. Low precision High accuracy Figure 2. Low precision Low accuracy white paper 03 One of the most common yet poorly understood concepts in instrumention is accuracy. The meaning of "accuracy" can vary widely and manufacturers' published accuracy specifications can often be very confusing. The proper interpretation of the word depends upon the means by which it was measured and the way in which the specification is to be used in a particular system. In fact, "accuracy" may be expressed through several different terms. Here are the most common, along with some questions that illustrate how each might be used: ACCURACY If the average of several outcomes were computed, how close to the desired outcome would the average of these outcomes be? PRECISION If several instances of the measurement action were taken, how consistent would the results be? RESOLUTION How fine an increment can the process make? Note that an instrument may have high resolution and but still have poor accuracy in relation to the desired outcome. REPEATABILITY If the same action were taken in the same way and under identical conditions as a previous action, how closely would the new outcome match the previous outcome of that same action? This often refers to a fluid circuit, as opposed to a specific component. Understanding the differences between these terms will help instrument designers interpret component specifications and understand the nature of component errors and possible remedies. Accuracy - Definitions and Misconceptions

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