1 / 20

Power Analysis in Grant Writing

Power Analysis in Grant Writing. Jill Harkavy-Friedman, Ph.D. Is there a difference?. Power is Knowledge. Power : How likely are you to detect an effect? Sample Size : n How many people will you need? Effect Size : σ (e.g., R 2 ) How much of a difference are you trying to detect?

heatherw
Download Presentation

Power Analysis in Grant Writing

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Power Analysis in Grant Writing Jill Harkavy-Friedman, Ph.D.

  2. Is there a difference?

  3. Power is Knowledge

  4. Power: How likely are you to detect an effect? • Sample Size: n How many people will you need? • Effect Size: σ (e.g., R2) How much of a difference are you trying to detect? • Significance Level (Type I Error): α How much of a risk are you willing to take of saying there is a difference when there none? • Type II Error: β or (1- α) • How much risk there is of saying that there is no difference when there is a difference.

  5. Sample Size: n • Depends on Nature of the question & statistic approach • Group differences: m1-m2=0 • Correlations: r=0 • Regression: R2=0 • Feasibility • Economic, staffing, recruitment • Power needed

  6. Effect Size: σ • Amount of difference want to detect • Based on previous literature: average SD • Based on pilot data: SD • Based on size of difference • Small: d=.20 • Medium: d=.50 • Large: d=.80 • Statistical difference ≠ Clinical difference

  7. Significance Level (Type I Error): α Normal Curve and Distribution of Sample Means 10.0 7.5 Count 5.0 2.5 2.5% 11% 34% 34% 11% 2.5% 0.0 3 4 5 6

  8. The larger the sample size the greater the power • The larger the effect size the greater the power • The larger the significance level the greater the power

  9. Power and Sample Size Table at α 1=.05

  10. Data Analysis comes first - power second • Determine your hypotheses • Determine your analyses • Determine the parameters for analysis by hypothesis (i.e., power, ES, α) • Conduct power analysis

  11. Power analysis will require: • Type of analysis • Sample size • Effect size • Significance level • Number of groups or factors • Plug in the numbers

  12. Considerations • What is your question? • What type of data do you have? • What are your hypotheses? • What are your resources? • Clinical vs. statistical significance • How will you present your data? • What you would like the news headline to be?

  13. SAGES Research Committee, August 2006 RE: Suggestions to Assist with Completing a Winning Application • Power Analysis: • In order to minimize the reporting of false-negative data, a power analysis should be performed for sample size determination. Power is the capability of a study to detect a difference if the difference really exists. A type II error occurs when a true difference exists between study populations but there are insufficient numbers of subjects to detect this difference. • Any grant submitted without one of the items below will not be eligible for review.

  14. Power analysis. Please provide the following data: alpha and beta, sample size needed in each group, what difference is expected. (Example: "A power analysis was performed with a beta of .20 and an alpha of 05. Assuming that a 10% difference exists between patient and control groups, 150 subjects will be needed in each arm. Thus the study would provide an 80% chance that a difference would be detected if one exists.") • If a power analysis in not appropriate for the submitted project, a statement should be included explaining why a power analysis is not appropriate for the study. • Consultation with a statistician is recommended. However, there are many statistical software programs available

  15. What to do when you need more power • Increase sample size • Reduce number of variables • Show your data graphically

  16. Power Analysis With n=400, alpha2 = .05, and a medium effect size (.30) the power will be >.99 for analyses of variance and .96 for zero order correlations120. The power for a regression analysis that includes 11 variables (i.e. sex, ethnicity, positive symptoms, negative symptoms, aggression, impulsivity, depression, premorbid adjustment, gene marker, family history and substance abuse) with n=400, alpha=.05 and a medium effect size (R2=.10) will be greater than .90. We do not anticipate that all 11 variables will contribute significantly to the model. With 6 variables, a more likely model, the power will be > .90. For the exploratory regression analyses conducted within the group of attempters (n=200) the power will be at least .80 to detect a medium effect size (R2=.10). For correlational studies among the genetic and biochemical measures (approximate n=100) the power will be .80 for a medium effect size (R2=.10) and .95 for a larger medium effect size (R2=.25). The increased sample size will now provide the power necessary to consider attempters and nonattempters separately. Analyses examining single attempters, multiple attempters and nonattempters (approximate sample sizes: 120, 80, 200) will still maintain adequate power (power>.70). Grant application by Jill M. Harkavy-Friedman, PhD

  17. Free Power and Sample Size Calculation • Cohen J. Statistical Power Analysis for the Behavioral Sciences (2nd edition). Hillsdale, New Jersey: Lawrence Erlbaum Associates, Publishers, 1988 • http://biostat.mc.vanderbilt.edu/twiki/bin/view/Main/PowerSampleSize • G*Power 2 • http://www.psycho.uni-duesseldorf.de/aap/projects/gpower/ (limited) • G*3 • http://www.psycho.uni-duesseldorf.de/abteilungen/aap/gpower3/

More Related