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194 Cards in this Set

  • Front
  • Back
statistics is
a set of mathematical procedures used to summarize or draw conclusions from data.
data are
are values representing the measurement
of one or more variables.
a variable is
anything that varies, or changes.
a score is
is a particular person’s value on a variable.
descriptive statistics are
procedures used to summarize and present data
inferential statistics are
procedures used to draw conclusions about a
population based on data from a sample
a population is
is the complete set of all individuals
of interest in a study. Usually, it is too large.
a sample is
is a manageable subset of the population
a parameter is
is a value that summarizes a
characteristic of the entire population.
a parameter is
is constant
• Because all the data of interest is used to calculate
the parameter, there is no additional data that can
change its value.
– But parameters are typically unknown.
a statistic is
is a value that summarizes a
characteristic of a sample.
– It is typically used to estimate the parameter of
a real or hypothetical population.
a statistic is
is variable
• Because different samples may contain different
scores from the population, statistics of the same
variable may differ between samples.
two types of categorical scales
nominal and ordinal
nominal scales are
• values label or identify observations (e.g., male/female).
• mathematical operations: = and not equals
ordinal scales are
• values rank order observations (e.g., letter grades)
• mathematical operations: = , not equals , < , and >
• intervals are not necessarily equal
two types of quantitative scales
interval and ratio
interval scales are
• values order observations into equally spaced intervals
(e.g., degrees Fahrenheit).
• mathematical operations: = , not equals, < , > , + , and –
• * and / not meaningful because no true zero point
ratio scales are
• have a true zero point (e.g., inches).
• mathematical operations: = , not equals, < , > , + , - , *, and /
discrete data are
– there are not an infinite number of meaningful values that
exist between any two neighboring values
– All categorical scales produce discrete data.
– Quantitative scales may produce discrete or continuous
data.
continuous data are
– there are an infinite number of meaningful values that
exist between any two neighboring values
– possible values only limited by precision of measurement
instrument
a real limit is
– the range of values within which the true value
of a continuous value exists

upper real limit = value + ½(measurement unit)

lower real limit = value – ½(measurement unit)
a measurement unit is
the lowest measurable value greater than zero
X and Y are
the scores on each variable
N is
total number of participants in an
entire study or population
n is
number of participants in a condition
of a study or a single sample
sigma is
summation (add up all the scores)
rules of priority
• all operations
– inside parentheses
– in the numerator and denominator
– under a square root

• exponentiation
• negation
• multiplication and
division

• addition and
subtraction
frequency (f)
– the number of times a value of a particular
variable occurs in the data
frequency distribution
– the complete set of frequencies for each value
of a variable
making a frequency table
If the data are nominal,
the order of the
categories is up to you.

Otherwise, the values of
the variable should be
listed from highest ( top)
to lowest (bottom).
cumulative frequency (cf)
the frequency of scores less
than or equal to a given value
proportion (p)
p = f/N
percentage (%)
% = 100(p)
cumulative percentage (c%)
– the percentage of
scores less than or
equal to a given
value

c% = 100(cf/N)
graphs
– X axis (abscissa) = categories or values of the X variable
– Y axis (ordinate) = the statistic being presented
bar graphs
– for categorical (nominal or ordinal) data
histograms
– for quantitative (interval or ratio) data
frequency polygons
– for quantitative (interval or ratio) data
normal distribution
– a symmetrical “bell-shaped” distribution in which
frequency steadily increases as values approach the
midpoint of the distribution
skew
the degree to which the distribution is unsymmetrical
positive skew
• most scores are on the low
end of the distribution
negative skew
• most scores are on the
high end of the distribution
modes
– the number of distinct “peaks”
– one distinct peak = unimodal
– two distinct peaks = bimodal
– more than two = multimodal
central tendency
– the typical score
mean
• arithmetic average of the scores
median
• middle score
mode
the most frequent score
notation for the mean
– μ (mu) for the population mean (usually unknown)
– M for a sample mean
mean is
is the balancing point of
a distribution
a deviation
is each score minus the
mean (X – M).
• The sum of the deviations always
equals zero.
The mean is sensitive to outliers
– extreme scores that skew the
distribution

– the mean is a poor measure of central
tendency when there are outliers
the median is not affected by
outliers
what do you do when the mean is sensitive to outliers?
find the median
the grand mean
the mean of several
sample means.
when to use the median instead of the mean
– If the data include outliers
– If the data include undetermined values
– If the distribution is open-ended
– If the data are ordinal
the mode is
Only measure of central tendency for nominal data
• Otherwise, not a good measure because distributions can have more than one distinct
mode or no mode at all
range
– upper real limit of the highest score – the lower real
limit of the lowest score
– whole numbers: highest score – the lowest score + 1
interquartile range
– the difference between the the first and third
quartiles
quartiles are
are values that divide a distribution of
scores into four equal parts

first quartile (Q1) divides bottom 25% from top 75%
• second quartile (Q2) divides bottom 50% from top
50%; same as the median
• third quartile (Q3) divides bottom 75% from top 25%
semi-interquartile range
– half the interquartile range

SIQR = (Q3 – Q1)/2
variance(σ2)
the average squared deviation
standard deviation(σ)
– the square root of the variance
populations vs. samples
A sample tends to have less variability than the population from
which it was drawn.
how is a statistic biased?
A statistic is biased if, on average, it underestimates
or overestimates its corresponding parameter.
how is a statistic unbiased?
Dividing by n – 1 results in an unbiased
estimate of the population variance and
standard deviation.
– A statistic is unbiased if, on average, it equals its
corresponding parameter.
degrees of freedom (df)
– number of values that are free to vary in a sample
– for a single sample with one variable, df = n – 1
what do the mean and standard deviation tell us?
The mean (μ) and standard deviation (σ) tells us the position of a score relative to other scores in the population.
raw scores
are the data directly observed from
the measurements
standardized scores
indicate the relative position
of the raw scores in a distribution
standardizing changes the scale how?
Standardizing changes the scale into units of
variability (i.e., standard deviations)
z-scores are what type of data?
interval data
problems with z-scores
– large z-scores still appear small
– a z-score of zero is average
– negative values are confusing
• Solution: convert z-scores by changing the mean and standard deviation
scatterplot
– a two dimensional plot of each subject based on
their scores from two variables
correlation
the linear relationship between two variables
positive correlation
as one variable increases, the other variable tends
to increase
negative correlation
as one variable increases, the other variable tends
to decrease
correlation direction
Whether a correlation is positive or negative
is called its direction
correlation magnitude
The accuracy of one variable’s prediction of
the other variable is called the magnitude.
Pearson product moment correlation
coefficient (r)
– measures the degree to which the scatter plot
forms a straight line
– ranges from
• –1 (perfect negative correlation)
• to 0 (no correlation)
• to 1 (perfect positive correlation)
– the sign indicates direction
– the number indicates magnitude
definitional formula of the pearson r
– the average cross-product of the z-scores
r 2 = coefficient of determination
the proportion of variance in one variable (Y) that
can be predicted from the other variable (X).
1 - r 2 = coefficient of nondetermination
the proportion of variance in one variable (Y) that
cannot be predicted from the other variable (X).
very strong pearson r means:
r is > or equal to .70

r2 is > or equal to .49
strong pearson r means:
r is greater than or equal to .50 - .69

r2 is greater or equal .25 - .48
moderate pearson r means:
r is greater or equal .30 - .49

r2 is greater or equal .09 - .24
weak pearson r means:
r is greater or equal .10 - .29

r 2 is greater or equal .01 - .08
very weak pearson r means:
r is less than or equal to .10

r2 is less than or equal .01
spurious correlations
are correlation coefficients
that are artificially high or low
outliers
anyone who is more than 3 standard deviations away from the mean, if so you can remove them from your data but you must report it
restricted range
example: harvard cuts off at 1300 on SATs
curvilinearity
when the relation between two variablesis better described as a curve
sample size
the larger the sample, the more reliable the correlation
n should be greater or equal to
30
Given any correlation, there are three
possible causal relationships.
X ------> Y

Y------> X

3rd factor ----> X and Y
in a standard normal distribution:
Approx. 68% of the distribution is within 1 σ of μ.

Approx. 95% of the distribution is within 2 σ of μ.

Approx. 100% of the distribution is within 3 σ of μ.
percentile rank (PR)
– the percentage (%) of scores less than or equal to a
given score (X) rounded to the nearest whole
number.
unit normal table
lists the proportion
of the standard normal distribution less than
and greater than any given z-score
Body p
is the larger proportion of the
distribution associated with a given z-score.
Tail p
is the smaller proportion of the
distribution associated with a given z-score.
percentile
– the score (X) at which a given percentage (%) of
the distribution is equal to or less than that score.
probability (p)
– the frequency (f) of an outcome relative to all possible outcomes (N)
– probability = proportion
– probability ranges from .00 to 1.00
– assumes random sampling

p = f/N
random sampling
each outcome or individual has an equal chance of
occurring or being selected from the population
sampling error
the discrepancy between a sample statistic and
the population parameter

Sampling error causes
variability in the
sample means.
sampling distribution (of the mean)
– the theoretical distribution of sample means if
all possible samples of equal size were selected
Mean of the sampling distribution
distribution
• Mean of the sampling distribution
– the expected value of any given sample mean
– on average, the sample mean will equal the mean
of the sampling distribution
standard error
– the standard deviation of the sampling distribution
– standard error is a measure of sampling error
as the standard error decreases
so does the average discrepancy
between the sample mean and
the population mean
central limit theorem rule 1
1. the mean of the sampling distribution equals
the population mean

thus, on average the sample mean equals the population mean
central limit theorem rule 2
the standard error is equal to the population
standard deviation divided by the square root
of the sample size

Standard error is an index of sampling error.
– Thus, as sample size (n) increases, sampling
error decreases.
– Therefore, the law of large numbers:
law of large numbers
The larger the sample, the more likely the sample
mean will equal the population mean
central limit theorem rule 3
The shape of the sampling distribution
approaches normal as sample size (n)
increases.
– The sampling distribution is practically normal
when…
• the population distribution is normal,
or
• the sample size (n) is greater than 30
research hypotheses
– a statement that outlines the predicted relationship
between the variables of interest

– e.g., rewards for good grades affect students’ IQ
statistical hypotheses
– two mutually exclusive mathematical propositions
about the parameter(s) of an unknown population
Null hypothesis (H0)
– states the expected value of a parameter (e.g., μ)
if the treatment has no effect ( i.e., the research hypothesis is false)
– e.g., “rewards do not affect students’ IQ” would
be written like so...
H0: μ = 100
where μ is the hypothesized mean of the
unknown population
Alternative hypothesis (H1)
– states the opposite of the null hypothesis
– e.g., “rewards do affect students’ IQ” would be
students IQ written like so...
H1: μ ≠ 100
hypothesis test
– procedure for estimating the probability that a
sample was drawn from a population if the null
hypothesis is true
– test whether the null is false, not whether the
alternative is true
deductive falsification
• a procedure in which evidence is sought leading to the
conclusion that a hypothesis is incorrect
• based on the logic of modus tollens
Modus tollens (denying the consequent)
– If A, then B (If there is fire, then there is oxygen.)
– Not B (There is no oxygen.)
– Therefore, not A (Therefore, there is no fire.)
• Affirming the consequent is not valid.
– If A, then B (If there is fire, then there is oxygen.)
– B (There is oxygen.)
– Therefore, A (Therefore, there is fire.)
Modus tollens applied to hypothesis testing
– If the hypothesis is true, then a prediction follows
(If H, then P).
– If the prediction is not true (Not P), then the
hypothesis is not true (Therefore, not H)
– If the prediction is true (P), then stating that the
hypothesis is true (Therefore, H) is not valid.
• Because we can only falsify a hypothesis, we
either reject the null or fail to reject the null
critical region
– the extreme values that are
unlikely be obtained if the null is
true
Alpha
– the probability of obtaining a
sample from the critical region.
– in the behavioral sciences, alpha
typically equals .05
the p-value
– the probability of selecting a
sample more extreme than the
current sample if the null hypothesis is true.
if p < a
• then the sample is in the critical region
• reject the null hypothesis
• the result is significant
if p > a
• then the sample is not in the critical region
• fail to reject the null hypothesis
• the result is not significant
two-tailed tests
– p and α are divided between the two tails of the
distribution
– Used for nondirectional hypotheses
non-directional hypotheses
research hypotheses that do not specify the direction
of the relationship between the variables
format for statistical hypotheses for two-tailed tests
H0: μ = value and H1: μ not equal value
one-tailed tests
– p and α are in one tail of the distribution
– Used for directional hypotheses
directional hypotheses
research hypotheses that specify the direction of the
relationship between the variables
• keywords: greater, more, higher, better, increases,
less, lower, worse, decreases
format for statistical hypotheses for one-tailed tests
H0: μ < value and H1: μ > value
H0: μ > value and H1: μ < value
research hypothesis for one-tailed test
Research hypothesis:
– Rewards increase student’s IQ
• Statistical hypotheses:
H0: μ < 100
H1: μ > 100
• α and p are in right tail
research hypothesis for one-tailed test
• Research hypothesis:
– Rewards decrease student’s IQ
• Statistical hypotheses:
H0: μ > 100
H1: μ < 100
• α and p are in left tail
Type I error
– rejecting the null hypothesis when it is true
– alpha ( α) is the probability of a Type I error
Type II error
– failing to reject the null hypothesis when it is
false

– beta (β) is the probability of committing a Type II error
power
1 – β
• the probability of rejecting the null hypothesis if it is
false
• power should be > .80
understanding power
look at notes
power increases as
the effect size increases.
effect size
is the difference between the population mean
based on H0 (μ0) and the actual population mean (μ1) .
power increases as
the standard error decreases
power increases as
the standard error decreases
– the standard deviation decreases
– the sample size increases
power increases as
α increases
power increases as
α increases
– but increasing alpha also increases the probability
of a Type I error
when sample sizes are small
When sample sizes are small (n < 30), the
sampling distribution may not be normal
when standard deviation is unknown
When σ is unknown, the standard error
cannot be calculated
student's t
Uses the sample standard deviation (s) to
estimate the standard error
using student's t
When you use the estimated standard error
to standardize the sample mean, the statistic
is called t instead of z.
t distribution
a symmetrical distribution with a mean of zero
that gets wider and flatter as df decrease

– When df = infinity, the
distribution is
perfectly normal
and t = z.
if |t| > tc then p < α
then reject the null hypothesis
Significance does not indicate ...
Significance does not indicate effect size.
– Remember, effect size is the difference between
the actual population mean and the population
mean based on the null hypothesis.
– Small effects can be significant if sample size is
large enough because larger samples have less
sampling error and thus more power.
estimating effect size
If, on average, the sample mean equals the
actual population mean, then M – μ is an
estimate of the effect size (where μ is based on
the null hypothesis)
cohen's d
is the standardized estimate of
effect size
large effect size (cohen's d)
greater than .80
medium effect size (cohen's d)
.20 - .80
small effect size (cohen's d)
less than .20
related samples t test
Variation of the single sample t test used for:
– within-subjects
- matched-subjects

• Null hypothesis states that, on average, the
difference between the scores is zero.
within-subjects (repeated measures) design
• a research design in which the individuals of a single
sample are measured more than once
matched-subjects design
• a research design in which individuals from one sample
are paired with individuals from another sample based on
one or more variables that the researcher wants to control
independent samples t test
Used for between-subjects designs

• The samples are drawn from two theoretical
populations.
• Null hypothesis states that the difference
between the means of the two populations is
zero.
between-subjects design
a research design in which the means of different
unmatched samples are compared (e.g., an
experimental sample and a control sample)
Sampling distribution of the difference
(between the means)
– the distribution of all possible differences between the means of all possible samples of a given size
the distribution of all possible differences between
– the mean of this distribution = 
1 - 
2
• if the null hypothesis is true, then the mean of this
distribution is equal to zero
– the standard deviation of this distribution is called
the standard error of the difference
Sampling distribution of the difference
(between the means)
Pairwise error rate (α)
– the probability of committing a Type I error
when comparing two means
Experimentwise (familywise) error rate
– the total probability of committing a Type I
error when performing more than one pairwise
comparison
• equal to 1 – (1 – α)c, where c = no. of comparisons
• probability of committing at least one Type I error
increases as the number of comparisons increase
Analysis of variance (ANOVA)
Analysis of variance (ANOVA)
– determines whether the variance between
groups significantly exceeds the variance
within groups due to error
Within-group variance (σ2
w)
Within-group variance (σ2
w)
– the total variance within each group
– variability in subjects responses due to error
• extraneous variables other than the treatment or
independent variable that affect subjects’ scores
Between-group variance (σ2
b)
– the variance among the means of each group
treatment variance (σ2t)
refers to the variability
in people’s responses due to the treatment or
independent variable only
Anova
If treatment variance = 0, then F = 1
• We reject the null if F is significantly greater than 1.
F distribution
an unsymmetrical distribution that becomes more
and more positively skewed as the degrees of
freedom within and the degrees of freedom
between decrease
total sum of squares (SST)
is the SS of all the scores
across all groups
total degrees of freedom
(dfT) is the df of all the
scores across all groups
Factorial design
is a research design in which
the effects of more than one factor on the
dependent variable are examined.
factor
A factor is the same as the treatment or independent
variable, or any categorical variable (e.g., gender)
that may be related to the dependent variable.
ANOVA is used to
ANOVA is used to analyze factorial designs
– A two-way ANOVA analyzes two factors, a threeway
ANOVA analyzes three factors, etc.
main effect of B
– What is the effect of B ignoring A?
– Does the mean time following the “persist”
instructions differ from the mean time following
the “do not persist” instructions?
Main effect of A
– What is the effect of A ignoring B?
– Does the mean time for low SE people differ
from the mean time for high SE people?
A x B interaction
– Does the effect of B depend on A?
– Does mean time difference between instructions
for low SE people differ from the mean time
difference between instructions for high SE
people?
Non-parallel lines usually
indicate ...
Non-parallel lines usually
indicate an interaction.
Parallel lines indicate...
Parallel lines indicate no
interaction
graphing factorial designs
look over in notes
Two-way ANOVA
Two-way Analysis of Variance
• Also called a kA x kB ANOVA, where...
kA is the number of levels of factor A
kB is the number of levels of factor B
kAkB = # of groups/conditions in the experiment
– For example
• a 2 x 2 ANOVA has 4 groups
• a 2 x 3 ANOVA has 6 groups
• a 3 x 3 ANOVA has 9 groups
• a 3 x 4 ANOVA has 12 groups, etc.
When reporting the results of an two-way ANOVA...
When reporting the results of an two-way
ANOVA, state whether each main effect and
the interaction were significant or not.
• Example:
– The 2 x 3 ANOVA revealed a significant main
effect for anger, F (1, 54) = 5.00, p < .05. The
main effect for cues was not significant, F (2, 54)
= 3.00, p > .05. The Anger x Cues interaction
was significant, F (2, 54) = 6.00, p < .05.
Parametric tests
Parametric tests
– inferential statistics that assume certain
characteristics of the population are true
– t tests and ANOVAs are parametric
Nonparametric tests
Nonparametric tests
– inferential statistics that make few or no
assumptions about the characteristics of the
population
Assumptions of parametric tests
– normality
• the population distribution is normal
– homogeneity of variance
• when comparing two or more groups, the population
variances of each group are equal
– data are quantitative (interval or ratio)
Why use parametric tests?
– Parametric tests have more power than
nonparametric tests.
– Generally, if sample sizes are large (N > 30), t
tests and ANOVAs are robust against violations
of normality and homogeneity of variance.
Robust
means that the statistical tests still perform well,
even though certain assumptions may not be true.
Why use nonparametric tests?
– If parametric assumptions have been violated and
sample sizes are small (N < 30), then parametric
tests may lead to an inflated probability of a Type I
error.
– If the data are categorical (nominal or ordinal),
parametric tests should not be used.
Chi-square Goodness of Fit Test
• nonparametric test of whether the observed
frequencies in each category conform to
(i.e., “fit”) the frequencies expected based
on the null hypothesis.
fo
is the observed frequency
fe
is the expected frequency
Is the x2 significant?
the distribution is
positively skewed,
and becomes flatter
and longer as
degrees of freedom
increase.
Chi-square Test for Independence
• nonparametric test of whether the observed
frequencies associated with two variables
are independent (i.e., unrelated).
Writing in APA Style

for chi-square test for independence
Extroverts were significantly more likely to conform
than introverts, x2(1, N = 60) = 4.15, p < .05.