Explain why it is important to look at measures of spread, and not just center, when comparing two distributions.

http://www.mathsteacher.com.au/year10/ch16_statistics/04_spread/24spread.htm

They summarise how close each observed data value is to the mean value.
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When rates are expressed as a quantity of 1, such as 2 feet per second or 5 miles per hour, they are called unit rates. if you have a multiple-unit rate such as 120 students for every 3 buses, and want to find the single-unit rate, write a ratio equal to the multiple-unit rate with 1 as the second term.