“Life, unlike a math textbook, doesn’t have all of the answers

in the back of the book.”Source lost

The Real World

• Technicians get paid a reasonable salary to follow recipes.

• Scientists get paid higher salaries to write recipes for technicians to follow.

Good Time to Review Definitions of Precision and Accuracy

• Precision– Random errors are small

• Give examples of random errors

– Results are extremely reproducible– Large number of significant digits can be justified– Experimenter is skilled

• Accuracy– Systematic errors are small

• Give examples of systematic errors

– Result reveals truth

• Experiments with large random errors– Social sciences

• Political polling• Economics• Opinion polls

– Astronomical measurements• Cosmic Ray research!• The value of the Hubble constant

• Experiments with large systematic errors– Poorly calibrated equipment– Consistent parallax– Voltmeter with low input impedance or

inaccurate components– Basically indefensible

A Digression on Error Analysis and Significant Digits

• The number of “significant digits” that you report in lab reports will be taken very seriously– This is “where the rubber hits the road”– If too small, your work will not be taken

seriously• You get no credit for claiming to have measured

the speed of light to be 3x1010 cm/sec in 2009!

– If too large, your work is indefensible and effectively fraudulent. (You are claiming something you have, in fact, not done.)

How Do You Determine How May Digits Are Really Justified?

• Hg manometer?– What are the sources of systematic error?– How could you “calibrate” your

measurements?– How precisely (or is it accurately?) can you

read the mercury levels?

• Other gauges– How do you calibrate them?– How precisely (or is it accurately?) can you

read them?