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## Precision, Accuracy, Validity, and Reliability

Recall from research methods that there measurements have these four characteristics. A measurement can be more or less precise - how fine a resolution or how many decimal places? A measurement can be accurate (read "near the true value") or inaccurate (aka "wrong"). A measurement can be valid - it measures what we say it measures - or not. And a measurement can be reliable - if you measure again using the same instrument you will get more or less the same result.

## Numbers to Know

Basic numbers we should know (in their most basic form)

Ten basic numbers a person in policy should have an intuitive feel for

WorldAbout 7 billion and U.S.About 310 million1 population; US legislature435 congress + 100 senators and California legislatureAssembly 80, Senate 402; number of California counties58; US GDP$13 trillion (thousand billion)3; diameter of earth8,000 miles; median household income in the US$50,0004

Basic sizes. Width of a finger or hand. Ceilings. Doors. Tables. Width of a car.

Orders of magnitude

Communicating Relative Magnitudes

Getting a feel for magnitudes

How to approximate a number. Eschew meaningless precision.
Round
Order of magnitude
Scientific notation
Call it something you can remember

If the universe were a baseball diamond type analogies.

Dimensional analysis
Guesstimate
Heuristic
Rule of thumb
Sanity test

How to carry worst/best case values through a calculation

## Exercises

How many hours did you spend on facebook last year?
How many golf balls will fit in this room?
About how much area would we need for a 250 car parking lot?

If the world energy consumption is about 5 x 1020 joules5 and US is about 1 x 1020 joules6, then what is the ratio of US per capita usage to that of the world as a whole?

(1)
\begin{align} USPC = \frac{1 \times 10^{20} joules} {3 \times 10^8 people} \end{align}
(2)
\begin{align} WPC = \frac{5 \times 10^{20} joules} {7 \times 10^9 people} \end{align}
(3)
\begin{align} \frac{USPC} {WPC} = \frac{\frac{1 \times 10^{20}} {3 \times 10^8}} {\frac{5 \times 10^{20}} {7 \times 10^9}} \end{align}
(4)
\begin{align} \frac{USPC} {WPC} = \frac{\frac{1 \times 10^{12}} {3}} {\frac{5 \times 10^{11}} {7}} \end{align}
(5)
\begin{align} \frac{USPC} {WPC} = \frac{1 \times 10^{12}} {3} \times {\frac{7} {5 \times 10^{11}}} \end{align}
(6)
\begin{align} \frac{USPC} {WPC} = \frac{10} {3} \times {\frac{7} {5} = \frac{70} {15}} = \frac{14} {3} \simeq 5 \end{align}

Announce rather than discard inconvenient uncertainty7

### References

Mitchell N Charity <ude.tim.scl|ytirahcm#ude.tim.scl|ytirahcm> A View from the Back of the Envelope
Wikipedia. "Back of the envelope calculation"
Louisiana Lessons. 1996. Classic Fermi Questions with annotated solutions
Wikipedia. Fermi Problem
mittechtv. 2010. BLOSSOMS - The Art of Approximation in Science and Engineering: How to Whip Out Answers Quickly
Lori Ann White The art of back-of-the-envelope calculations: Students estimate their way through pop culture problems to learn a life skill. Symmetry Online Magazine

#### Back of the Envelope Calculations

Notes by , 04 Dec 2014 18:13
03 Dec 2014 21:31
in discussion Hidden / Per page discussions » Sourcing

Wikipedia Editors. Sourcing Disambiguation Page.

References by , 03 Dec 2014 21:31
03 Dec 2014 20:53
in discussion Hidden / Per page discussions » Time and Project Management

Assay My Week Exercise
Billable Hours Exercise

Tools
Fancy and Simple
https://basecamp.com
https://asana.com/

Resources by , 03 Dec 2014 20:53

"When to Use Which User-Experience Research Methods" by CHRISTIAN ROHRER on October 12, 2014

References by , 03 Dec 2014 19:24
02 Dec 2014 20:25