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Statistical Decision Making

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Presentation on theme: "Statistical Decision Making"— Presentation transcript:

1 Statistical Decision Making

2 History of the Student’s t Distribution and the t Test
Guinness and statistics

3 Hypothesis Testing State the null hypothesis
State the alternate hypothesis Select the level of significance Collect and summarize the sample data Refer to the criterion for evaluating the sample evidence Make a decision to discard/retain the null hypothesis

4 Type I and Type II Errors
Reality Null is true Null is false Accept null No error Type II error Reject null Type I error

5 Possible Causes of Error
Type I Measurement error Lack of random sample p value too liberal (p = .10) Investigator bias Improper use of one-tailed test Type II Measurement error Lack of sufficient power (N too small) P value too conservative (p = .01) Treatment effect not properly applied

6 The Alpha and p Levels p value - probability with which the difference in means could occur by chance Alpha () level - p value set prior to the experiment Indication of risk willing to take in making a wrong decision to reject the null hypothesis Smaller p values do not indicate greater importance of findings Only indicates a smaller probability that the findings occurred by chance

7 Two-Tailed Test When direction of outcome is uncertain we use the null hypothesis Ho is tested with a two-tailed test If t does not reach p = .05, we know sample mean falls within the normal curve that includes 95% of all possible differences Accept Ho with a 5% chance of being wrong 5% rejection area divided between the 2 tails of the curve Each area is 2.5% of the area under the curve

8 One-Tailed Test Use when direction of difference is know
Not sure what the size of difference is Most likely trying to test H1 Difference to be tested is always positive Only use positive side of normal curve

9 Why Use Statistical Decision Making?
Make decisions about differences between groups Not necessarily that the 2 groups are different from each other Did the two groups come from different populations?

10 The t Test Used when standard deviation of the population is not known
Need to calculate the standard error of the mean estimated from a sample Using SEM we can determine odds that a sample is representative of the population it is drawn from

11 The Independent t Test Method comparing independent samples
Comparing means of 2 samples Need to determine the standard error of the difference (SED) from the SEM of each sample t Test becomes

12 Independent t Test Software packages Calculate t value and p value
p typically = .05 as standard for rejecting null hypothesis 5 times in 100

13 Assumptions for the t Test
t Test produces reasonably reliable results Robust measure 4 assumptions Population from which sample are drawn is normally distributed Sample or samples are randomly selected from the population Homogeneity of variance Variance of one group should not be more than twice as large as the variance of the other Data must be parametric Interval or ratio measurement scale

14 The Correlated t Test Standard formulas for calculating t assume no correlation between groups Dependent samples assume relationship (correlation) between scores from same group of subjects Posttest is partially dependent on pretest score

15 Correction for Correlated Samples
Need to make adjustment to denominator T Test becomes

16 Reading the Statistical Output
Figure 12-2 Independent t Test Figure 12-3 Dependent t Test Paired samples

17 Statistical Power Power - Ability of a test to correctly reject a false null hypothesis Since critical t values are lower for one-tailed test, it is considered more powerful at a given p value 1- is area of power

18 Power Power is dependent on 4 factors
Z level set by researcher The difference between the 2 means being compared The standard deviation of the 2 groups Determines the spread of the curve The sample size (N) of the 2 groups Only N and Z are under your control

19 Calculating Power Power is calculated by determining Z, converting it to a percentile, and adding this percent to the 50% of the curve to the right of the experimental curve t = Z + Z Z = t - Z Use table A to determine percentile for Z add that value to 50% = 1-

20 Sample Size The only factor easily manipulated in power is N
How large does N need to be to produce a given power? Complicated calculation This is the last step

21 Statistical vs. Clinical Significance
Which one is important? Effect size holds the answer

22 Effect Size .30 is low .50 is moderate .80 is high Example from book
Should be obvious to clinician Example from book ES = 2.68/5.01 ES = 0.53

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