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Descriptive Statistics
Used to describe or summarize sets
of data to make them more
– measures of central tendency
mean, median, mode
– measures of variability
range, standard deviation
– measures of association
correlation coefficient
Measures of Central
 What
is the average family
income above?
Mean - the arithmetic average
– Total sum of all scores divided by the number
of scores
Median - the center score
– 50th percentile score, half of the scores are
above and half are below
Mode - the score that occurs the most
Measures of Variability
(amount of variation in the data – how
similar or diverse the scores are)
Range - the difference between the
highest and lowest score in a set of
– Not always reliable – extreme scores
(outliers) can create a deceptively large
Measures of Variability
Standard deviation - reflects the
average distance between every
score and the mean
– Better gauge of whether scores are
packed together or dispersed
– Large std. deviations signify that
scores are more scattered, therefore
the mean is not terribly typical
Normal Distribution
Normally distributed variables produce
the familiar symmetric, bell-shaped
curves obtained when large numbers of
observations are made on a single
Curves can be skewed in a positive or
negative direction depending on the
position of the long tail (outliers), not
the position of the bulge
Normal Distribution
Skewed Distribution
Measures of Association
Correlation Coefficient 
Often we measure more than one
– Grade point and SAT score
Are they related?
 Correlation statistic is a way to find
Inferential Statistics
Descriptive statistics summarize a data
 We often want to go beyond the data
 Is the world at large like my sample?
 Are my descriptive statistics
 Inferential statistics give probability
that the sample is like the world at
Statistics and Probability
Probability means how likely
something is
How likely are results like mine to
occur by chance?
Statistical Significance
How likely it is that a study’s results
occurred by chance
(t-test or chi-square test)
– Statistically significant – reflects the real
world rather than chance – p≤.05
A result would be considered significant if it
would be expected to occur by chance 5 or
fewer times in 100 repetitions of the study.
– Not significant – results reflect chance –
Inferential Statistics
When is an observed difference reliable?
 Representative samples are better than
biased samples.
 Less-variable observations are more
reliable then those that are more
 More cases are better than fewer.
Sources of Bias
Biased sample - when the members of
a sample differ in a systematic way
from the larger population the
researcher is interested in
– interested in all voters
– contact by telephone
– biased sample - lower economic groups may not
own telephones
Ethical Issues in
Psychological Research
Right to privacy
 Informed consent
– use of deception
Animal rights
– Is there justification for discomfort or harm a
research procedure may produce?
APA publishes ethical guidelines