Accuracy and PrecisionAccuracy and Precision

A MEASURE of your

SUCCESS!

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40

30

20

10

IT’S TRUE!IT’S TRUE!

Accuracy:~ indicator of the closeness of a

measurement to its ‘true’ or ‘accepted’ value

Dependent upon:

~ how carefully the measurement is taken

~ nature of the instrumentation used

Right On!Right On!

Precision:~ describes the agreement

among several results measured in the same way

~ ‘reproducibility’ of measurements

Tell Me!Tell Me!High or Low accuracy and precision

Low accuracy

Low precisionLow accuracy

High precision

High accuracy

High precision

Lab things to consider

• Procedures v. Data table

• Using the balance

• Volume v. volume

• What is a meniscus?

• Collecting data v. analyzing data

• Are all those zeros necessary?

Significant figures (or digits)

• Things to consider:D = m/V m= 14.3216 g V = 2.00 cm3

D = ?

Exact numbers (no uncertain digit)• Counted numbers (students in the class)• Numerical relationships (1 km = 1000 m)

Inexact numbers (last digit is always uncertain digit)• Result from measurement ( 25.3 mL in grad. Cylinder)• Depend upon what measuring device is being used

Uncertain digit

- Last digit in a measurement that can be read with reasonable reliability

RULES TO LIVE BY…

All nonzero digits are significant

96 g ? Sig figs.

61.4 cm ? Sig figs.

4.720 mL ? Sig figs.

8.00 g ? Sig figs.

One or more final zeros used after the decimal point are always significant

Zeros between other significant digits are always significant

5.03m ? Sig figs.

3.008 g ? Sig figs.

.00054 cm ? Sig figs.

0.01354 mg ? Sig figs.

Zeros used solely for spacing the decimal point are not significant. (place holders)

1. Addition and Subtraction:

~ the answer must be rounded to the same number of digits to the right of the decimal point as there are in the measurement with the smallest number of digits to the right of the decimal

EX. 2.89 m + 0.0043 m = 2.8943 m

= 2.89 m (correct # of sig figs)

2. Multiplication and Division:

• the product or quotient should be rounded to the same number of significant figures as in the measurement with the fewest total number of significant figures

Ex. 3.5293 g x 34.2 g = 120.70206 g2

= 121 g (correct # of sig figs)