stor 155, section 2, last time hypothesis testing –assess strength of evidence with p-value...
TRANSCRIPT
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Stor 155, Section 2, Last Time• Hypothesis Testing
– Assess strength of evidence with P-value
• P-value interpretation:
– Yes – No
– Gray – level
– 1 - sided vs. 2 - sided “paradox”
bdryHHconclusivemoreorsawwhatPvalueP A&| 0
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Reading In Textbook
Approximate Reading for Today’s Material:
Pages 400-416, 424-428, 450-471
Approximate Reading for Next Class:
Pages 485-504
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Hypothesis Testing, III
A “paradox” of 2-sided testing:Can get strange conclusions
(why is gray level sensible?)
Fast food example: suppose gathered more data, so n = 20, and other results are the same
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Hypothesis Testing, III
One-sided test of:
P-value = … = 0.031
Part 5 of http://stat-or.unc.edu/webspace/postscript/marron/Teaching/stor155-2007/Stor155Eg24.xls
Two-sided test of:
P-value = … = 0.062
000,20$:0 H
000,20$: AH
000,20$:0 H
000,20$: AH
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Hypothesis Testing, III
Yes-no interpretation:
Have strong evidence
But no evidence !?!
(shouldn’t bigger imply different?)
000,20$
000,20$
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Hypothesis Testing, IIINotes:i. Shows that yes-no testing is different
from usual logic(so be careful with it!)
ii. Reason: 2-sided admits more uncertainty into process
(so near boundary could make a difference, as happened here)
iii. Gray level view avoids this:(1-sided has stronger evidence,
as expected)
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Hypothesis Testing, III
Lesson: 1-sided vs. 2-sided issues need careful:
1. Implementation
(choice does affect answer)
2. Interpretation
(idea of being tested
depends on this choice)
Better from gray level viewpoint
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Hypothesis Testing, III
CAUTION: Read problem carefully to distinguish between:
One-sided Hypotheses - like:
Two-sided Hypotheses - like:
:.:0 AHvsH
:.:0 AHvsH
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Hypothesis TestingHints:• Use 1-sided when see words like:
– Smaller– Greater– In excess of
• Use 2-sided when see words like:– Equal– Different
• Always write down H0 and HA – Since then easy to label “more conclusive”– And get partial credit….
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Hypothesis Testing
E.g. Text book problem 6.34:
In each of the following situations, a
significance test for a population mean,
is called for. State the null hypothesis,
H0 and the alternative hypothesis, HA
in each case….
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Hypothesis TestingE.g. 6.34aAn experiment is designed to measure the
effect of a high soy diet on bone density of rats.
Let = average bone density of high soy rats = average bone density of ordinary rats
(since no question of “bigger” or “smaller”)
O
OHSH :0OHSAH :
HS
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Hypothesis TestingE.g. 6.34bStudent newspaper changed its format. In a
random sample of readers, ask opinions on scale of -2 = “new format much worse”, -1 = “new format somewhat worse”, 0 = “about same”, +1 = “new a somewhat better”, +2 = “new much better”.
Let = average opinion score
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Hypothesis TestingE.g. 6.34b (cont.)
No reason to choose one over other, so do two sided.
Note: Use one sided if question is of form: “is the new format better?”
0:0 H
0: AH
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Hypothesis TestingE.g. 6.34cThe examinations in a large history class are
scaled after grading so that the mean score is 75. A teaching assistant thinks that his students have a higher average score than the class as a whole. His students can be considered as a sample from the population of all students he might teach, so he compares their score with 75.
= average score for all students of this TA75:0 H 75: AH
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Hypothesis Testing
E.g. Textbook problem 6.36
Translate each of the following research
questions into appropriate and
Be sure to identify the parameters in each
hypothesis (generally useful, so already
did this above).
0H AH
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Hypothesis TestingE.g. 6.36aA researcher randomly divides 6-th graders
into 2 groups for PE Class, and teached volleyball skills to both. She encourages Group A, but acts cool towards Group B. She hopes that encouragement will result in a higher mean test for group A.
Let = mean test score for Group A = mean test score for Group BAB
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Hypothesis TestingE.g. 6.36a
Recall: Set up point to be proven as HA
BAH :0
BAAH :
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Hypothesis TestingE.g. 6.36bResearcher believes there is a positive
correlation between GPA and esteem for students. To test this, she gathers GPA and esteem score data at a university.
Let = correlation between GPS & esteem
0:0 H
0: AH
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Hypothesis TestingE.g. 6.36cA sociologist asks a sample of students
which subject they like best. She suspects a higher percentage of females, than males, will name English.
Let: = prop’n of Females preferring English = prop’n of Males preferring English
Fp
MF ppH :0
MFA ppH :
Mp
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Hypothesis Testing
HW on setting up hypotheses:
6.35, 6.37
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Hypothesis Testing
Connection between Confidence Intervals
and Hypothesis Tests:
Reject at Level 0.05 P-value < 0.05
dist’n
Area < 0.05
0.95 margin of error
mX
mXmX , CIinnot 95.0
X
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Hypothesis Testing & CIs
Reject at Level 0.05
Notes:
1. This is why EXCEL’s CONFIDENCE function uses = 1 – coverage prob.
2. If only care about 2-sided hypos, then could work only with CIs
(and not learn about hypo. tests)
CIinnot 95.0
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Hypothesis Testing & CIs
HW: 6.71
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Hypothesis Testing
The three traps of Hypothesis Testing
(and how to avoid them…)
Trap 1: Statistically Significant is different
from Really Significant
(don’t confuse them)
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Hypothesis Testing Traps
Trap 1: Statistically Significant is different
from Really Significant
E.g. To test a painful diet program, 10,000
people were put on it. Their average
weight loss was 1.7 lbs, with s = 73.
Assess “significance” by hypothesis testing.
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Hypothesis Testing Traps
Trap 1: Statistically Significant is different
from Really Significant
See Class Example 25: Trap 1http://stat-or.unc.edu/webspace/postscript/marron/Teaching/stor155-2007/Stor155Eg25.xls
P-value = 0.0099
Strongly Statistically
Significant
Careful: Is this practically significant?
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Hypothesis Testing Traps
Trap 1, e.g: Is this practically significant?
NO! Not worth painful diet to lose 1.7 lbs.
Resolution: Hypo. testing resolves question:
Could observed results be due to
chance variation?
Answer here is no, since n is really large.
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Hypothesis Testing Traps
Trap 1, e.g: Is this practically significant?
Answer here is no, since n is really large.
But this is different from question:
Do results show a big difference?
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Hypothesis Testing Traps
Trap 2: Insignificant results do not mean
nothing is there,
Only: Didn’t have strong enough data to
actually prove results.
E.g. Class 25, Trap 2http://stat-or.unc.edu/webspace/postscript/marron/Teaching/stor155-2007/Stor155Eg25.xls
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Hypothesis Testing Traps
Trap 3: Try enough tests, and you will find “something” even where it doesn’t exist.
Revisit Class Example 21, Q4http://stat-or.unc.edu/webspace/postscript/marron/Teaching/stor155-2007/Stor155Eg23.xls
We saw about 5% of CIs don’t cover.
So, (using CI – Hypo Test connection), expect about 5% of tests to choose HA, (and claim
“strong evidence) even when H0 true.
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Hypothesis Testing Traps
Strategies to avoid Trap 3:
1. Scientific Method:
Form Hypothesis tests once.
2. For repeated tests: use careful adjustments:
(beyond scope of this course)
Get help if needed
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Hypothesis Testing Traps
HW: 6.74 (about 50)
6.82
(0.382, 0.171, 0.0013)
6.83, 6.84
(0.0505, 0.0495)
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Some stuff for next time
(next slides)
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Hypothesis Testing
View 3: level testing
Idea: instead of reporting P-value, choose a
fixed level, say 5%
Then reject H0, i.e. find strong evidence…
When P-value < 0.05 (more generally )
(slight recasting of yes-no version of testing)
HW: 6.53 (careful, already assigned above)
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Hypothesis Testing
View 4: P-value shows only half of the
decision problem:
Graphical Illustration: Truth
H0 true HA true
Test Result: Choose H0
Choose HA
Correct Type II Error
Type I Error
Correct
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Hypothesis Testing
View 4: Both sides of decision problem:
Small P-values Small Type I error
What about Type II error?
(seems part of problem)
a. Simplistic Answer: Don’t care because
have put burden of proof on HA.
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Hypothesis Testing
View 4: What about Type II error?
b. Deeper Answer: Does matter for test
sensitivity issues, and test power issues
E.g. how large a sample size is needed for a
given test power (treated above)?
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Hypothesis Testing
View 4: Terminology:
P{Type I Error} is called level of test
1 - P{Type I Error} is called specificity
1 – P{Type II Error} is called power of test
1 – P{Type II Error} is called sensitivity
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And now for something completely different…
A statistician’s view on politics…
Some Current Controversial Issues:
• North Carolina State Lottery
• Replace Social Security by Individual Retirement Plans
Debate is passionate, (natural for complex and important issues)
But what is missing?
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And now for something completely different…
Review Ideas on State Lotteries,
from our study of Expected Value
Not an obvious choice because:
• Gambling is (at least) unsavory:– Religious objections– Some like it too much– Destroys some lives
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And now for something completely different…
State Lotteries, not an obvious choice:• The only totally voluntary tax:
– Nobody required, unlike all other taxes– Money often used for education– Good or bad, given state of economy???
• Highest tax burden on the poor– Poor enjoy playing much more– Higher taxes on poor better for society???– Tendency towards “rich get richer”???
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And now for something completely different…
What about Individual Retirement Plans:
Main Benefit:
On average individual investments
return greater yields
than government investments
So can we conclude:
• “Overall we are all better off”???
• Since more total money to go around?
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And now for something completely different…
Very common mistake in this reasoning:
• Notice “on average” part of statement
• Should also think about variation about
the average???
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And now for something completely different…
Variation about average Issue 1:
• Should think of population of people
• Average is over this population
• Except some to do great
• And expect some to lose everything
• What will the percentage of losers be?
• What do we do with those who lose all?
• What will that cost?
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And now for something completely different…
Variation about average Issue 2:
• Also are averaging over time
• Overall gains of stock market happen only over this average
• Some need $$$ when market is down
• How often will this happen?
• How do we deal with it?
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And now for something completely different…
Main concept I hope you carry away from this course:
Variation is a fundamental concept
• Look for it
• Think about it
• Ask questions about it
(Vital to informed citizenship)
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And now for something completely different…
Australian joke about Variation:
Did you hear about the man who drowned in a lake with average depth 6 inches?
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And now for something completely different…
Australian joke about Variation:
He understood “average”, but not variation about the average
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And now for something completely different…
Really have such lakes?
Yes, in Australia
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And now for something completely different…
Suggestions of such issues (politics, controversy…) for discussion are welcome….
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Hypothesis Testing
Other views of hypothesis testing:
View 2: Z-scores
Idea: instead of reporting p-value (to assess statistical significance)
Report the Z-score
A different way of measuring significance
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Hypothesis Testing – Z scores
E.g. Fast Food Menus:
Test
Using
P-value = P{what saw or m.c.| H0 & HA bd’ry}
000,20$:0 H
000,20$: AH
10,400,2$,000,21$ nsX
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Hypothesis Testing – Z scores
P-value = P{what saw or or m.c.| H0 & HA bd’ry}
rybdXP '|000,21$
000,20$|000,21$ XP
102400$
000,20$000,21$
nsX
P
317.1 ZP
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Hypothesis Testing – Z scores
P-value
This is the Z-score
Computation: Class E.g. 24, Part 6http://stat-or.unc.edu/webspace/postscript/marron/Teaching/stor155-2007/Stor155Eg24.xls
Distribution: N(0,1)
317.1 ZP
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Hypothesis Testing – Z scores
P-value
So instead of reporting tail probability,
Report this cutoff instead,
as “SDs away from mean $20,000”
HW: 6.67 , but use NORMDIST, not Table D
317.1 ZP
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Sec. 7.1: Deeper look at Inference
Recall: “inference” = CIs and Hypo Tests
Main Issue: In sampling distribution
Usually is unknown, so replace with an estimate, .
For n large, should be “OK”, but what about:
• n small?
• How large is n “large”?
nNX /,0~
s
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Unknown SD
Approach: Account for “extra variability in the approximation”
Mathematics: Assume individual
I.e.
• Data have mound shaped histogram
• Recall averages generally normal
• But now must focus on individuals
s ,~ NX i
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Unknown SD
Then
Replace by , then
has a distribution named:
“t-distribution with n-1 degrees of freedom”
nNX /,~
1,0~ N
n
X
sn
sX
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t - Distribution
Notes:
1. n is a parameter (like ) that controls “added variability from approximation”
View: Study Densities,
over degrees of freedom…http://stat-or.unc.edu/webspace/postscript/marron/Teaching/stor155-2007/EgTDist.mpg
,,, ps
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t - Distribution
Comments on movie
• Serious differences for n <= 10
(since approximation is bad)
• Very little difference for n large
(since approximation really good)
• Rule of thumb cutoff: many say
“negligible difference for n > 30”
• Cutoffs more informative than curves
s
s
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t - Distribution
Nice Alternate View:Webster West (U. So. Carolina) Applet:
http://www.stat.sc.edu/~west/applets/tdemo1.html
Notes:– Big change for small degrees of freedom– Looks very normal for large degrees of
freedom– I would overlay with Normal…
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t - Distribution
Notes:
2. Careful: set “degrees of freedom” =
= n – 1 (not n)
• Easy to forget later
• Good to add to sheet of notes for exam
HW: 7.21 a
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t - Distribution
Notes:
3. Must work with standardized version of
i.e.
• No longer can plug mean and SD
• into EXCEL formulas
• In text this was already done,
• Since need this for Normal table calc’ns
nsX X