Download Ways of Describing Data

The trouble with being average…
• Measuring the center
of a distribution:
– Mean (pg 39)
– Median (pg 40)
– Mode
1
x   xi
n
Contrasting Mean and Median
• Skewed distributions: Mean and median
measure different things – you must
decide which is appropriate.
• Median divides the data set in half –
regardless of shape.
• Mean is the “balance point” if you are
dealing with a distribution density (next
lecture).
The Quartiles (not a stats singing group!)
• Quartiles divide data sets (ie distributions)
into quarters. The 1st quartile is the
median of the first half of the set, the 3rd
quartile is the median of the last half.
(skill testing question: where is the 2nd quartile?)
• Did you read your text? What’s a pth
percentile?
• Refer to examples 1.15 and 1.16 to help
consolidate your understanding
Measuring the Spread in Data
• Two lab groups of physics students, using identical equipment, made
the following measurements of the distance between nodes in a
sound experiment.
Mean = 30.7
Mean = 31.72
Median = 30.65
Median = 30.95
StDev = 0.389
StDev = 1.798
Standard Deviation…

(x

x
)
i
n 1
2
You said this wasn’t a math course!
How a Statistician Cures a Fever…
• Your forehead is burning up – you
have a fever. No problem, just stick
your feet in a bucket of ice-water. On
average your temperature is just fine!
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