Power
We want high power in our study. At least 0.8. Power – probability of rejecting a false null hypothesis. Probability of detecting a difference between conditions when one exists. We want high power in our study. At least 0.8. What does Power of 0.8 mean? ch12(1)
Factors that Effect Power 1. Design of the study: Within Subjects is more powerful than Between subjects. 2. Reliability of the measures: If there is a lot of noise in the measures the study will have less power. Q: If I wanted to measure a persons basket throwing ability would I get a better measure by counting Whether they sink a basket on one try. The total number they sink out of 10 tries. The total number they sink out of 50 tries.
3) Effect Size – Correlation between IV and DV. Tells you how well you can predict the DV, if you know the participant’s IV condition. e.g., If I know whether a person is male or female, how much does that improve my ability to predict their math ability. Cohen’s (1992) Criteria. Effect size or .10 is small, .30 is moderate and .50 is large. ch12(1)
The larger the Effect Size is the more Power the study has. Very powerful studies can find significant (not due to chance) effects that are very small. Studies with low power may be unable to detect even large effects of the IV. ch12(1)
4) Alpha Level: Chance we are willing to take of making a Type I error. If we change alpha to .10, we would correctly accept a lot more comparisons as significant, but we will also greatly increase our chance of making a type one error. ch12(1)
5) Sample size The larger the sample the more powerful the design. Why 5) Sample size The larger the sample the more powerful the design. Why? The larger the sample the more accurate the estimates (statistics) and therefore the cleaner the comparison.
Sample Size, Power and Effect Size are related in such a way that if you know two of these values for a study, you can determine the third (from a chart) for a given alpha level. ch12(1)
↑ Sample Size ↑ Power ↑ Effect Size ↑ Power ↑ Alpha ↑ Power ch12(1)