Download Statistics

Statistics
Recording the results from our
studies.
Descriptive Statistics
• Just describes sets of data.
• You might create a
frequency distribution.
• Frequency polygons or
histograms.
Measures of Central Tendency
• Mode – score that occurs most often
EX: 3,5,5,7,5,4,9,6,8,7,10 5 is the mode
• Median – score in the halfway point
EX: 3,4,5,5,5,6,7,7,8,9,10  6 is the median
• Mean – average score
EX: 3+5+5+7+5+4+9+6+8+7+10=69
69/11  6.27 is the mean
Mean, Median, Mode
• The weekly salaries of six employees at
McDonalds are $140, $220, $90, $180, $140, $200.
– For these six salaries, find:
• (a) the mean
• (b) the median
• (c) the mode
Mean, Median, Mode
• Answers
Mean:
90+ 140+ 140+ 180 + 200 + 220 =$ 161.67
6
Median:
90,140,140,180,200,220
The two numbers that fall in the middle need to be averaged.
140 + 180 = 160
2
Mode:
90,140,140,180,200,220
The number that appears the most is 140
Let’s say you are looking to
move to a new place….
• The average income of a family in the new
city is 70,000 – not too bad….
• However….
• 26 families make less than 70,000
• Only 3 families make more than 70,000
Skewed Distributions
15 20 25 30 35 40 45 50
90
475
70
Mode Median
One Family
Mean
Income per family in thousands of dollars
710
Distributions
• Outliers skew
distributions.
• If group has one high
score, the curve has a
positive skew (contains
more low scores)
• If a group has a low
outlier, the curve has a
negative skew (contains
more high scores)
Measures of Variation
Measures of Variability
• Range: distance from
highest to lowest scores.
• Standard Deviation:
the variance of scores
around the mean.
• The higher the variance
or SD, the more spread
out the distribution is.
• Do scientists want a big or
small SD?
Calculating the Standard
Deviation
Data: 2,2,4,8
1. Figure out the Mean: 16 4 = 4
2. Take mean & figure out difference from
each point: 2-4=-2, 2-4=-2, 4-4=0, 8-4=4
3. Square each difference:4,4,0,16
4. Add together: 4+4+0+16=24
5. Divide by #: 24 4 = 6  This is the
variance
6. Take square root: 2.45
Normal Distribution
• In a normal
distribution, the mean,
median and mode are
all the same.
Normal Distribution
A normal distribution
is a bell shaped curve.
A standard deviation of 15 accounts for about 68% of responses.
Statistical Significance
• How likely it is that an obtained result
occurred by chance
• Odds must be less than 5% (p<.05) in order
for results to be statistically significant
• This means that your results are 95%
(p>.95) a result of your IV