Transcript Day 12 Class Slides - School of Information

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INF397C
Introduction to Research in Information
Studies
Spring, 2005
Day 12
R. G. Bias | School of Information | SZB 562BB | Phone: 512 471 7046 | [email protected]
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Context
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Where we’ve been:
– Descriptive statistics
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Frequency distributions, graphs
Types of scales
Probability
Measures of central tendency and spread
z scores
– Experimental design
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The scientific method
Operational definitions
IV, DV, controls, counterbalancing, confounds
Validity, reliability
Within- and between-subject designs
– Qualitative research
• Gracy, Rice Lively
– Inferential statistics
• Dillon – standard error of the mean, t-tests
• Doty – Chi square
R. G. Bias | School of Information | SZB 562BB | Phone: 512 471 7046 | [email protected]
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Context (cont’d.)
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• Where we’re going:
– More descriptive statistics
• Correlation
– Inferential statistics
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Confidence intervals
Hypothesis testing, Type I and II errors, significance level
t-tests
Anova
– Which method when?
– Cumulative final
R. G. Bias | School of Information | SZB 562BB | Phone: 512 471 7046 | [email protected]
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Standard Error of the Mean
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• So far, we’ve computed a sample mean
(M, X bar), and used it to estimate the
population mean (µ).
• One thing we’ve gotten convinced of (I
hope) is . . . larger sample sizes are
better.
– Think about it – what if I asked ONE of you,
what School are you a student in? Versus
asking 10 of you?
R. G. Bias | School of Information | SZB 562BB | Phone: 512 471 7046 | [email protected]
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Standard Error (cont’d.)
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• Well, instead of picking ONE sample, and using that
mean to estimate the population mean, what if we
sampled a BUNCH of samples?
• If we sampled ALL possible samples, the mean of the
means would equal the population mean. (“µM”)
• Here are some other things we know:
– As we get more samples, the mean of the sample means gets
closer to the population mean.
– Distribution of sample means tends to be normal.
– We can use the z table to find the probability of a mean of a
certain value.
– And most importantly . . .
R. G. Bias | School of Information | SZB 562BB | Phone: 512 471 7046 | [email protected]
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Standard Error (cont’d.)
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• We can easily work out the standard deviation of the
distribution of sample means:
– SE = SM = S/SQRT(N)
• So, the standard error of the mean is the standard
distance that a sample mean is from the population
mean.
• Thus, the SE tells us how good an estimate our
sample mean is of the population mean.
• Note, as N gets larger, the SE gets smaller, and the
better the sample mean estimates the population
mean.
• Hold on – we’ll use SE later.
R. G. Bias | School of Information | SZB 562BB | Phone: 512 471 7046 | [email protected]
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Two methods of making
statistical inferences
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• Null hypothesis testing
– Assume IV has no effect on DV; differences we
obtain are just by chance (error variance)
– If the difference is unlikely enough to happen by
chance (and “enough” tends to be p < .05), then we
say there’s a true difference.
• Confidence intervals
– We compute a confidence interval for the “true”
population mean, from sample data. (95% level,
usually.)
– If two groups’ confidence intervals don’t overlap, we
say (we INFER) there’s a true difference.
R. G. Bias | School of Information | SZB 562BB | Phone: 512 471 7046 | [email protected]
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Remember . . .
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• Earlier I said that there are two ways for
us to be confident that something is true:
– Statistical inference
– Replicability
• Now I’m saying there are two avenues of
statistical inference:
– Hypothesis testing
– Confidence intervals
R. G. Bias | School of Information | SZB 562BB | Phone: 512 471 7046 | [email protected]
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Effect Size
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• How big of an effect does the IV have on
the DV?
• Remember, two things that make it hard
to find a difference are:
– There’s a small actual difference.
– There’s a lot of within-group variability (error
variance).
– (Demonstrate with moving distributions.)
R. G. Bias | School of Information | SZB 562BB | Phone: 512 471 7046 | [email protected]
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Effect Size (cont’d.)
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• From S, Z, & Z: “To be able to observe the
effect of the IV, given large within-group
variability, the difference between the two
group means must be large.”
• Cohen’s d = (µ1 – µ2)/ σ
• “Because effect sizes are presented in
standard deviation units, they can be used to
make meaningful comparisons of effect sizes
across experiments using different DVs.”
R. G. Bias | School of Information | SZB 562BB | Phone: 512 471 7046 | [email protected]
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Effect Size (cont’d.)
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• When σ isn’t known, it’s obtained by
pooling the within-group variability
across groups and dividing by the total
number of scores in both groups.
• σ = SQRT {[(n1-1)S12 + (n2-1) S22]/N}
• And, by convention:
– d of .20 is considered small
– d of .50 is considered medium
– d of .80 is considered large
R. G. Bias | School of Information | SZB 562BB | Phone: 512 471 7046 | [email protected]
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Effect Size example
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• Let’s look at the heights of men and
women.
• Just for grins, intuitively, what you say –
small, medium, or large difference?
• µwomen = 64.6 in. µmen = 69.8 in. σ = 2.8 in.
• d = (µ1 – µ2)/ σ = (69.8 – 64.6)/2.8 = 1.86
• So, very large difference. Indeed, one
that everyone is aware of.
R. G. Bias | School of Information | SZB 562BB | Phone: 512 471 7046 | [email protected]
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t-tests
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• Remember the z scores:
– z = (X - µ)/σ
– It is often the case that we want to know “What
percentage of the scores are above (or below) a
certain other score”?
– Asked another way, “What is the area under the
curve, beyond a certain point”?
– THIS is why we calculate a z score, and the way we
do it is with the z table, on p. 306 of Hinton.
• Problem: We RARELY truly know µ or σ.
R. G. Bias | School of Information | SZB 562BB | Phone: 512 471 7046 | [email protected]
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t-tests (cont’d.)
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• So, typically what we do is use M to estimate µ and s
to estimate σ. (Duh.) (Note: When we estimate σ
with s, we divide by N-1, which is degrees of freedom.)
• Then, instead of z, we calculate t.
• Hinton’s example on p. 64 is for a t-test when you
have a null hypothesis population mean (µ0). (That is,
you want to test if your observed sample mean is
different from some value.)
• Hinton then offers examples in Chapter 8 of related
(dependent, within-subjects) and independent
(unrelated, between-subjects) t-tests.
• S, Z, & Z’s example on p. 409 is for a t-test to compare
independent means.
R. G. Bias | School of Information | SZB 562BB | Phone: 512 471 7046 | [email protected]
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Formulae
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- For a single mean(compared with µ0):
- t = (M - µ)/(s/SQRTn)
- For related (within-subjects) groups:
- t = (M1 – M2)/s M1 – M2
- Where s M1 – M2 = (sx1 – x2)/SQRTn
- See Hinton, p. 83
- For independent groups:
- From S, Z, & Z, p. 409, and Hinton, p. 87
- t = (M1 – M2)/s M1 – M2
– Where s M1 – M2 = SQRT [(S12/n1) + (S22/n2)]
– See Hinton, p. 87
» Will’s correction – The minus sign in the numerator of the
formula at the top of page 87 should be a minus sign. Also two
formulas down. Hinton has it right by the bottom of the page.
R. G. Bias | School of Information | SZB 562BB | Phone: 512 471 7046 | [email protected]
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Steps
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• For a t test for a single sample
– Restate the question as a research hypothesis and a null hypothesis
about the populations.
– Determine the characteristics of the comparison distribution.
• The mean is the known population mean.
• Compute the standard deviation by:
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Calculate the estimated population variance (S2 = SS/df)
Calculate the variance of the distribution of means (S2/n)
Take the square root, to get SE.
Note, we’re calculating t with N-1 df.
• Determine the cutoff sample score on the comparison distribution at
which the null hypothesis should be rejected.
– Decide on an alpha and one-tailed vs. two-tailed
– Look up the critical value in the table
– Determine your sample’s t score: t = m- µ / SE
– Decide whether to reject or not reject the null hypothesis. (If the
observed value of t exceeds the table value, reject.)
R. G. Bias | School of Information | SZB 562BB | Phone: 512 471 7046 | [email protected]
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Steps
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For a t test for dependent means
– Restate the question as a research hypothesis and a null hypothesis about
the populations.
– Determine the characteristics of the comparison distribution.
• Make each person’s score into a difference score. From here on out, use difference
scores.
• Compute the mean of the difference scores.
• Assume a population mean of 0: µ = 0.
• Compute the standard deviation of the difference scores:
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Calculate the estimated population variance (S2 = SS/df)
Calculate the variance of the distribution of means (S2/n)
Take the square root, to get SE.
Note, we’re calculating t with N-1 df.
• Determine the cutoff sample score on the comparison distribution at which the null
hypothesis should be rejected.
– Decide on an alpha, and one-tailed vs. two-tailed
– Look up the critical value in the table
– Determine your sample’s t score: t = m - µ / SE
– Decide whether to reject or not reject the null hypothesis. (If the observed
value of t exceeds the table value, reject.)
R. G. Bias | School of Information | SZB 562BB | Phone: 512 471 7046 | [email protected]
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Steps
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• For a t test for independent means
– Same as for dependent means, except the value
for SE is that squirrely formula on Hinton, p. 87.
– Basically, here’s the point. When you’re comparing
DEPENDENT (within-subject, related) means, you
can assume both sets of scores come from the
same distribution, thus have the same standard
deviation.
• But when you’re comparing independent (betweensubject, unrelated) means, you gotta basically average the
variability of each of the two distributions.
R. G. Bias | School of Information | SZB 562BB | Phone: 512 471 7046 | [email protected]
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Three points
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• df
– Four people, take your choice of candy.
– One df used up calculating the mean.
• One or two tails
– Must be VERY careful, choosing to do a one-tailed
test.
• Comparing the z and t tables
– Check out the .05 t table values for infinity df (1.96
for two-tailed test, 1.645 for one-tailed).
– Now find the commensurate values in the z table.
R. G. Bias | School of Information | SZB 562BB | Phone: 512 471 7046 | [email protected]
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Significance Level
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• Remember, two ways to test statistical significance –
hypothesis tests and confidence intervals.
• With confidence intervals, if two groups yield data and
their confidence intervals don’t overlap, then we
conclude a significant difference.
• In hypothesis testing, if the probability of finding our
differences by chance is smaller than our chosen
alpha, then we say we have a significant difference.
• We select alpha (α), by tradition.
• Statistical significance isn’t the same thing as practical
significance.
R. G. Bias | School of Information | SZB 562BB | Phone: 512 471 7046 | [email protected]
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Type I and Type II Errors
World
Our
decision
Reject the
null
hypothesis
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Null
Null
hypothesis is hypothesis is
false
true
Correct
decision
Type I error
(α)
Fail to reject Type II error Correct
the null
(β)
decision
hypothesis
(1-β)
R. G. Bias | School of Information | SZB 562BB | Phone: 512 471 7046 | [email protected]
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Power of the Test
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• The power of a statistical test refers to its
ability to find a difference in distributions
when there really is one there.
• Things that influence the power of a test:
– Size of the effect.
– Sample size.
– Variability.
– Alpha level.
R. G. Bias | School of Information | SZB 562BB | Phone: 512 471 7046 | [email protected]
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