Transcript A = true value

Indicate the length up to one
estimated value….
A.
B.
C.
Convert
•
•
•
•
2,780 ml in Liter
0.098 mg in µg
2.00 m in mm
0.04 L in ml
Measure the volumes on stations 1-8
• You need at least 1 measured and at least
1 estimated value in your measurements
Evaluating Scientific Equipment
Measurement are evaluated based on
the following parameters
1. Accuracy:
How close are measurements to an ‘ideal/true
value’?
2. Precision:
How close are measurements to each other?
Other Statistical Key Terms
True Value:
An ideal/perfect measurement, a reference
value
Note: does often not exist in exploratory
research!
Average/Mean:
Arithmetic Mean: sum of all measurements
divided by number of measurements (n)
Target Practice Analogy
• What’s the true value in pictures A-C?
• Identifiy which one you think is accurate/precise
A
B
C
NOTE: A good measurement should be highly
accurate and highly precise!!! Which one is it?
Formulas:
• Accuracy (% Error)
A=
true value – average value x 100%
true value
• Precision (% Range)
P = highest value – lowest value x 100%
average value
Example: A lab tech has to calibrate a balance with
10.0 g weight samples. He makes 4 repeat
measurements: 9.6 g, 12.5g, 10.3 g and 9.8 g. Calculate
accuracy and precision of the balance…..
• Accuracy: (% Error)
true value – average value x 100%
true value
Find the following:
True Value ____
10.0g
Average Value ____
9.6 +12.5+10.3+9.8=42.2 = 10.55g
4
10 g – 10.55 x 100% = 5.5% no negative % please!!!
10 g
• Precision (% Range)
highest value – lowest value x 100%
average value
• Find
Highest value ___
Lowest value ____
Average value ____
12.5 g – 9.6 g x 100% = 27.5%
10.55g
• Accuracy: 5.5%
• Precision 27.5%
Note: the larger the %, the worst the quality!
Result statement: Balance is more accurate than
precise!!!
Think: Why accuracy alone is not a good indicator?
Values can average out to be close to the true value
(high accuracy), but precision will reveal it!
How to graph?
Accuracy and Precision of Balance with 10 g weights
g
10
5
A: 0.055 x 10.55 = +/-0.58
P: 0.275 x 10.55 = +/-2.90
Statistical tools used for research data
(no true value)
Variance:
Average of squared differences from the mean
Standard Deviation:
Square root of the Variance