1. Statistical significance & the Normal Curve

2. What tools have we used? Measures of Central Tendency
Mean, median, mode
Measures of Variation
Range, standard deviation, variance

3. When is a difference reliable?
When the sample is REPRESENTATIVE, not BIASED
Random sample, random assignment
When the observations are less variable, rather than more variable
Are there major outliers? Etc.
More cases, rather than fewer
Once we know sample is representative, you want more cases
Difficult to bring clout to findings of 3 people vs. 50,000 people

4. Statistical significance
“a statistical criterion for rejecting the assumption of no differences in a particular study
The likelihood that a result or relationship is caused by something other than mere random chance
Statistical hypothesis testing is traditionally employed to determine a
"p-value" representing the probability that random chance could explain the result
In general, a 5% or lower p-value is considered to be statistically significant
Psychologists look for a probability of 5% or less that the results are do to chance
This means there is a 95% chance the results are not due to chance

5. Frequency distributions
a mathematical function showing the number of instances in which a variable takes each of its possible values.
A visual showing how many times a number occurs
What would this look like on a bar graph?
How do you know the MODE?

6. The NORMAL CURVE
“the symmetrical bell-shaped curve that describes the distribution of many psychical and psychological attributes. Most scores fall near the average, and fewer and fewer scores lie near the extremes.”
% RULE

7. Normal curve
Gives us basis for comparison to “norms” or representative groups of people
Ex. IQ tests, heights, mental aptitudes
Scores are a certain standard deviation from the mean (Z-scores)

8. Normal curve As you get further to extremes, less scores occur
Z-score of 1 OR -1, 34%
Z-score of 2 OR -2, 13.5%
Z-score of 3 OR -3, 2.5%
As you get further to extremes, less scores occur
1 standard deviation off the mean (below and above) = 68% total, 34% each way
1-2 standard deviations away from the mean (either way) = 95% total, 47.5%
3 standard deviations away from the mean (either way) = 99.7%, 49.85% each
More than 3 standard deviations
away = 0.3% total, 0.15% each way

9. Normal curve practice What percent of scores fall when Z = +1?
What percent of scores fall within -1 and -2 standard deviations below the mean?
What percent of scores fall 0 to 3 standard deviations above the mean?