Accuracy and Precision in Measurement | Difference Between Accuracy & Precision

Accuracy and precision are fundamental concepts in measurements, crucial for fields like physics, engineering, and medicine. Understanding these concepts helps improve the reliability of measurements and ensures correctness in scientific and technical work.

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Accuracy

Definition

Accuracy refers to how close a measured value is to the true or accepted value of a physical quantity. It indicates correctness.

Examples of Accuracy

• If a clock shows the time as 3:00 PM and it is actually 3:00 PM, the clock is accurate.
• If a thermometer reads 100°C while the actual temperature is 99.9°C, it is considered accurate.

Accuracy Formula

Accuracy is determined using the percent error formula:

Percent Error = {(Measured Value – True Value)/True Value} × 100

This formula gives us the accuracy as a percentage. The less the percent error the more accurate the value is.

Types of Accuracy

1. Point Accuracy: Accuracy at a specific point on the instrument’s scale.
2. Accuracy as Percentage of Scale Range: Measured as a fraction of the instrument’s range.
3. Accuracy as Percentage of True Value: How close the measured value is to the true value.

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Precision

Definition

Precision refers to how consistently a measurement produces the same result under the same conditions. It measures repeatability and reliability.

Examples of Precision

• If you weigh an object five times and get 4.6 kg each time, your measurements are precise.
• A machine cutting metal pieces to exactly 5.00 cm repeatedly exhibits precision.

Types of Precision

1. Repeatability: Consistency of measurements taken by the same instrument under the same conditions.
2. Reproducibility: Consistency when different people use different instruments over extended periods.

Precision Formula

Precision is measured using the standard deviation formula: Standard Deviation (Precision) = √(∑(Measurement −Mean)²​​/ Number of measurements)

A lower standard deviation indicates higher precision.

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Difference Between Accuracy and Precision

Aspect	Accuracy	Precision
Definition	Closeness to the true value	Consistency of repeated measurements
Dependence	Depends on the true value	Independent of accuracy and true value
Example	A scale showing your true weight	A scale showing the same weight every time
Illustration	Darts hitting the bullseye	Darts landing close to each other but not necessarily on the bullseye
Measurement Error	Error is the difference from the true value	Error is the spread of repeated values

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FAQs

Q1. Can a measurement be precise but not accurate?
Yes, if measurements are consistently incorrect but close to each other.

Q2. Can a measurement be accurate but not precise?
Yes, if measurements are close to the true value but vary widely.

Q3. How can accuracy and precision be improved?
By using high-quality instruments, minimizing environmental interference, and calibrating tools regularly.

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MCQs with Answers

1. Which of the following best defines accuracy?
   • A) Closeness of measured values to each other
   • B) Closeness of a measured value to the true value ✅
   • C) Difference between measured values
   • D) None of the above
2. What is the main difference between accuracy and precision?
   • A) Accuracy is about correctness; precision is about consistency ✅
   • B) Precision is about correctness; accuracy is about consistency
   • C) Both mean the same thing
   • D) None of the above
3. A scale consistently gives a reading 5 kg less than the true weight. It is:
   • A) Accurate but not precise
   • B) Precise but not accurate ✅
   • C) Both accurate and precise
   • D) Neither accurate nor precise

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Conceptual Questions with Answers

1. Why is accuracy important in scientific measurements?
   Accuracy ensures that the measurements reflect the true value, which is crucial for scientific experiments and calculations.
2. How does precision impact industrial manufacturing?
   High precision in manufacturing ensures uniformity and quality in mass production.
3. Can a device be both accurate and precise?
   Yes, an instrument that consistently gives readings close to the true value is both accurate and precise.

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Do You Know?

• Accuracy in medical tests determines correct diagnosis and treatment.
• Astronomers use precision measurements to detect exoplanets.
• Sports science relies on precision timing for race results.

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Worksheet

Answer the following:

1. Define accuracy and precision with examples.
2. Differentiate between repeatability and reproducibility.
3. If a thermometer reads 38.5°C for a patient whose actual temperature is 39°C, calculate its percent error.

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Test Paper (Total Marks: 20)

Section A (1 mark each)

1. Define accuracy.
2. Give an example of precision.

Section B (2 marks each)

3. Explain the difference between accuracy and precision.
4. Why is reproducibility important in scientific experiments?

Section C (3 marks each)

5. A student measures a metal rod five times and gets readings: 10.1 cm, 10.2 cm, 10.1 cm, 10.0 cm, and 10.2 cm. Is this measurement precise? Why?
6. Discuss the significance of standard deviation in precision measurements.

Section D (5 marks)

7. Describe different types of accuracy and their applications in real life.

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Important Points for Quick Revision

• Accuracy = Closeness to the true value.
• Precision = Consistency of repeated measurements.
• Accuracy is determined by percent error.
• Precision is measured using standard deviation.
• Repeatability and reproducibility define precision.
• Accurate instruments minimize systematic errors.
• Precise instruments minimize random errors.

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Test Your Knowledge (Quiz)

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