Units of Measurement and Accuracy vs. Precision Vocabulary qualitative qualitative quantitative quantitative standard standard accuracy accuracy precision precision Imperial system Imperial system metric system metric system SI units SI units “Target with Four Faces” – 1955 Jasper Johns

Measurement A lot of science is qualitative (observations that do not involve measurement.) Examples: Classifying organisms Interpreting rock strata Understanding the Skeletal System

Measurement However, a very important aspect of science is that much of it is quantitative (observations that involve measurements) Examples: Determining the amount of CO 2 in the atmosphere Measuring the age of the earth Calculating the amount of energy produced by uranium

Standardization In order for scientists to communicate quantitative measurements, there must be standardized units (units that are agreed upon between scientists.)

Imperial (or English or Standard) System Imperial units are the units we use most of the time here in the US. What are some examples? Length: ft, mi, yd, in, fathoms, furlongs, rods, etc. Mass: lb, oz, ton, dram, pennyweight, grain, etc. Volume: gal, pt, qt, cp, fl.oz., tsp, tbls, gill, peck, bushel, etc.

Imperial (or English or Standard) System To convert between these units require conversion factors. For instance: 1 foot = _____ inches 1 gallon = _____ quarts 1 min = _____sec 1 mile = _____ feet

Imperial (or English or Standard) System More conversion factors (and this is just for volume!)

Imperial (or English or Standard) System Even though the units are standardized, the complicated and inconsistent nature of the Imperial System is why only 3 countries out of 192 in the world use it. Countries that use the Imperial system: The United States Liberia Myanmar (Burma)

SI (International System) All 189 other countries use the more consistent and logical SI units. They were adopted in 1960 by the General Conference of Weights and Measures and are founded on some base units: length: meter (m) mass: kilogram (kg) time: seconds (s) and some others we won’t cover right now

SI (International System) What are some other SI units? length: m, km, cm, mm, etc. mass: kg, g, cg, mg, etc. time: s, ms, ks, cs, etc. volume: L, mL, kL, cL, etc.

SI (International System) What are some other SI units? length: m, km, cm, mm, etc. mass: kg, g, cg, mg, etc. time: s, ms, ks, cs, etc. volume: L, mL, kL, cL, etc. Notice that each type of measurement can have the same prefixes: kilo, centi, milli, etc.

SI (International System) That’s what makes the SI units so appealing. 1 meter = 1000 millimeters 1 gram = 1000 milligrams 1 liter = 1000 milliliters 1 second = 1000 milliseconds The prefixes are consistent no matter what measurement you are making and all conversions are based on factors of 10.

SI Prefixes

However these are the only ones you will need to know.

When making measurements Scientists don’t just need to know about units. They need to use them correctly and make accurate and precise measurements.

Imagine a Dart Board If you were to throw a dart at the dart board, what you consider an accurate throw?

Imagine a Dart Board Which of these darts would you consider accurate? 1 2 3

Imagine a Dart Board #1 is accurate. Accuracy: the closeness of a measured value to an accepted value. 1

Imagine a Dart Board Because the center of the board is the accepted value for success, dart #1 is accurate. 1

If you read the thermometer and recorded a value of 102 o, is your reading accurate? Why or why not? It would not be accurate because the boiling point of water is 100 o C at sea level and lower at higher elevations. Imagine some boiling water

Now let’s talk about precision Precision has to do with how closely multiple measured values agree with each other.

Imagine a dart board Would you say that the three darts on the dart board are precise?

Imagine a dart board No, they are not precise because they are not close to one another. They are inconsistent.

Comment on the accuracy and precision of the following darts They are not accurate or precise.

Comment on the accuracy and precision of the following darts They are both accurate and precise.

Comment on the accuracy and precision of the following darts They are precise but not accurate.

Comment on the accuracy and precision of the following darts The average is accurate (perhaps) but they are not precise.

Back to the water Three students each perform three trials to find the boiling point of water at sea level. Their data is shown below. Trial 1Trial 2Trial 3 Student 199 o 100 o Student 294 o Student 393 o 98 o 95 o

Back to the water Which student was both accurate and precise? Trial 1Trial 2Trial 3 Student 199 o 100 o Student 294 o Student 393 o 98 o 95 o Student #1

Back to the water Which student performed a good experiment but probably had a faulty thermometer? Trial 1Trial 2Trial 3 Student 199 o 100 o Student 294 o Student 393 o 98 o 95 o Student #2

Back to the water Which student probably had multiple variables in his lab between trials? Trial 1Trial 2Trial 3 Student 199 o 100 o Student 294 o Student 393 o 98 o 95 o Student #3