Effect sizes - Sonoma State University · 4/17/2014 1 Effect sizes • r, R2 (adjusted...
Transcript of Effect sizes - Sonoma State University · 4/17/2014 1 Effect sizes • r, R2 (adjusted...
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Effect sizes
• r, R2 (adjusted R‐squared), η2 (partial η2), Cohen’s D
• Eta‐squared and Cohen's d provide two differenttypes of effect size and both may be appropriateand useful in reporting the results of ANOVA.
• Eta‐squared indicates the % of variance in the DV attributable to a particular IV.
• Cohen's d indicates the size of difference between two means in standard deviation units.
• η2= SSbetween / SStotal = SSB / SST– = proportion of variance in Y explained by X– = Non‐linear correlation coefficient– = proportion of variance in Y explained by X
• ranges between 0 and 1.• Interpret as for r2 or R2; a rule of thumb (Cohen):
– .01 ~ small– .06 ~ medium– .14 ~ large
• The eta‐squared column is not provided by SPSS, however, it is available in V 18.0. It can also be calculated manually: = Between‐Groups Sum of Squares / Total Sum of Squares.
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Statistical power
Power -> .10 .20 .30 .40 .50 .60 .70 .80 .90Effect size |.01 21 53 83 113 144 179 219 271 354.06 5 10 14 19 24 30 36 44 57.15 3 5 6 8 10 12 14 17 22
If you think that the effect is small (.01), medium, (.06) or large (.15), and you want to find a statistically significant difference defined as p<.05, this table shows you how many participants you need for different levels of “sensitivity” or power.
Statistical power is how “sensitive” a study is detecting various associations (magnification metaphor)
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Statistical power
What is adequate power? .50 (most current research).80 (recommended)
How do you know how much power you have? Guess work
Two ways to use power:1. Post hoc to establish what you could find2. Determine how many participants need
What determines power?
1. Number of subjects
2. Effect size
3. Alpha level
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How increase power?
1. Increase region of rejection to p<.10
2. Increase sample size
3. Increase treatment effects
4. Decrease within group variability
• Power = 1 ‐ type 2 error
• Type 1 and Type 2 errors
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Type 1 and Type 2 error
Type I Error
Rejecting the null hypothesis when it is in fact true is called a Type I error. (p=.03, p=.23)
Type II Error
Not rejecting the null hypothesis when in fact the alternate hypothesis is true is called a Type II error.
Putting concepts together
Truth
(for population studied)
Null
Hypothesis
True
Null
Hypothesis
False
Decision
(based on
sample)
Reject Null
HypothesisType I Error
Correct
Decision
Fail to reject
Null
Hypothesis
Correct
DecisionType II Error
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Putting concepts togetherTruth
Not Guilty Guilty
Verdict
Guilty
Type I Error ‐‐ Innocent
person goes to jail (and
maybe guilty person goes
free)
Correct Decision
Not Guilty Correct Decision
Type II Error ‐‐
Guilty person goes
free
An example
Null hypothesis is false Null hypothesis is true
Reject null hypothesis Merit pay works and we know it
We decided merit pay worked, but it doesn’t.
TYPE 1
Do not reject null hypothesis We decided merit pay does not work but it does.
TYPE 2
Merit pay does not work and we know it.
Type 1 error: think something is there and there is nothingType 2 error: think nothing is there and there is
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An example
Imagine the following research looking at the effects of the drug, AZT, if any, on HIV positive patients. In others words, does a group of AIDs patients given AZT live longer than another group given a placebo. If we conduct the experiment correctly ‐ everything is held constant (or randomly distributed) except for the independent measure and we do find a different between the two groups, there are only two reasonable explanations available to us:
From Dave Schultz:
Null hypothesis is false
Null hypothesis is true
Reject null hypothesis
Do not reject null hypothesis
• Educational consultants were hired by a progressive California school district to study the effectiveness of competitive vs. cooperative learning. The school district was known for its highly competitive philosophy, for which it was criticized despite its successful student test scores on national assessments. The educational consultants researched several school districts nationwide and in Japan. They found that school districts that operated with a cooperative model had significantly higher test scores and higher student satisfaction than the completive schools (i.e., investigators rejected the null hypothesis). The completive school district is strongly considering a bond measure in the next election to fund a change to the cooperative model with estimated costs of approximately 250 million dollars.
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• Investigators are researching a new vaccine to prevent the onset of AIDS in HIV positive subjects. One hundred HIV positive subjects volunteered to participate. Half of the subjects (N=50) were randomly assigned to Condition 1, which received the experimental vaccine, while the other half (N=50) were assigned to Condition 2, which received a placebo. Medically the vaccine results looked very encouraging, but statistically significant differences between the groups were not attained. The researchers failed to reject the null hypothesis (p<.01) and the public is denied possible treatment.
Study feature Practical way of raising power
Disadvantages
Predicted difference Increase intensity of experimental procedures
May not be practical or distort study’s meaning
Standard deviation Use a less diverse population
May not be available, decreases generalizability
Standard deviation Use standardized,controlled circumstances of testing or more precise measurement
Not always practical
Sample size Use a larger sample size Not practical, can be costly
Significant level Use a more lenient level of significance
Raises alpha, the probability of type 1 error
One tailed vs. two tailed test
Use a one-tailed test May not be appropriate to logic of study
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Outcome statistically significant
Sample Size Conclusion
Yes Small Important results
Yes Large Might or might not have practicalimportance
No Small Inconclusive
No Large Research H. probably false
Experiment 4: U.S. Undergraduates
11.5
22.5
33.5
44.5
55.5
66.5
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Prof. uncaring Prof. caring
Pro
fess
or's
tru
stw
orth
ines
s
Prof. competentProf. incompetent
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Confidence intervals
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• Write down the phenomenon you'd like to test. Let's say you're working with the following situation: The average weight of a male student in ABC University is 180 lbs.You'll be testing how accurately you will be able to predict the weight of male students in ABC university within a given confidence interval.
• Select a sample from your chosen population. This is what you will use to gather data for testing your hypothesis. Let's say you've randomly selected 1,000 male students.
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• Calculate your sample mean and sample standard deviation.Choose a sample statistic (e.g., sample mean, sample standard deviation) that you want to use to estimate your chosen population parameter. A population parameter is a value that represents a particular population characteristic. Here's how you can find your sample mean and sample standard deviation:– To calculate the sample mean of the data, just add up all of the
weights of the 1,000 men you selected and divide the result by 1000, the number of men. This should have given you the average weight of 186 lbs.
– To calculate the sample standard deviation, you will have to find the mean, or the average of the data. Next, you'll have to find the variance of the data, or the average of the squared differences from the mean. Once you find this number, just take its square root. Let's say the standard deviation here is 30 lbs. (Note that this information can sometimes be provided for you during a statistics problem.)
• Choose your desired confidence level. The most commonly used confidence levels are 90 percent, 95 percent and 99 percent. This may also be provided for you in the course of a problem. Let's say you've chosen 95%.
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• Calculate your margin of error. You can find the margin of error by using the following formula: Za/2 * σ/√(n). Za/2 = the confidence coefficient, where a = confidence level, σ = standard deviation, and n = sample size. This is another way of saying that you should multiply the critical value by the standard error. Here's how you can solve this formula by breaking it into parts:– To find the critical value, or Za/2: Here, the confidence level is 95%.
Convert the percentage to a decimal, .95, and divide it by 2 to get .475. Then, check out the z table to find the corresponding value that goes with .475. You'll see that the closest value is 1.96, at the intersection of row 1.9 and the column of .06.
– Take the standard error, take the standard deviation, 30, and divide it by the square root of the sample size, 1,000. You get 30/31.6, or .95 lbs.
– Multiply 1.96 by .95 (your critical value by your standard error) to get 1.86, your margin of error.
• State your confidence interval. To state the confidence interval, you just have to take the mean, or the average (180), and write it next to ±and the margin of error. The answer is: 180 ±1.86. You can find the upper and lower bounds of the confidence interval by adding and subtracting the margin of error from the mean. So, your lower bound is 180 ‐ 1.86, or 178.14, and your upper bound is 180 + 1.86, or 181.86.– You can also use this handy formula in finding the confidence interval: x̅ ± Za/2 * σ/√(n). Here, x ̅represents the mean.