Accuracy and Precision Accuracy and Precision A MEASURE of your SUCCESS!

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! Precision: ~ describes the agreement among several results measured in the same way ~ ‘reproducibility’ of measurements

Tell Me! High or Low accuracy and precision Low accuracy Low precision Low 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= g V = 2.00 cm 3 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 cm? Sig figs mL ? Sig figs 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 g? Sig figs cm? Sig figs 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 m m = 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 g x 34.2 g = g 2 = 121 g (correct # of sig figs)