1. Lesson 6
Normal and Skewed Distribution
Type one and Type two errors

2. Level of significance and errors
Level of significance accepted in psychology? %
% Too lenient
0.01 – 1% Too stringent but highly significant
0.1 – What is the problem with accepting the experimental hypothesis?
0.01 – what is the problem with rejecting the experimental hypothesis and accepting the null?

3. Type 1 & 2 Errors
There is a possibility that errors may have been made:
Type 1 Error
Deciding to reject the null when actually the results was due to chance or some other factor.
Type 2 Error
Deciding to retain the null when actually the result was caused by the IV.
Too Low
P = <0.10
Type 1 Error more likely
Too High
P = <0.001
Type 2 Error more likely

4. Normal and Skewed Distribution
Small samples – central tendency and standard deviation  useful statistics
Larger samples – useful to examine overall distribution formed by the data

5. Normal and Skewed Distribution
If mean, median and mode are the same/similar and focus around he middle set of scores (median) then there is normal distribution
Multi-modal – not normally distributed
When mean, median and mode are not similar – distribution will be skewed
Mainly below mean – negative skew
Mainly above mean – positive skew
Interval Data only

6. Interval. Data that are measured on some kind of scale, often temporal (e.g., the days of the week, hours of the day) where the differences between adjacent scale numbers are equal.
Ordinal. Elements of the data describe properties of objects or events that are ordered by some characteristic (e.g., how would you rank oranges as a snack food compared to tomatoes?) The order of the objects does not, however, provide any information about the distance along the continuum between any two adjacent items.
Nominal. Categories. This is simply putting items together without ordering or ranking them
Level of measurement

7. Distribution – only considered for interval data
So that mathematical calculations can be carried out
When scores are clustered around the mean and when the mean median and mode are similar
Normal distribution
Positively skewed distribution
Negatively skewed distribution
When scores are clustered below the mean and the mode shows that
When scores are clustered above the mean and the mode shows that

8. Normal distribution, Negatively or positively skewed scores?
Mean=?
Median=?
Mode=?
2. Provide a title
Frequency for the………

9. Normal Distribution
Bell shaped curve
Mean, median and mode should be aligned around the mid -point
Tail ends shouldn’t meet the horizontal axis
We can estimate the % of people that fall under the curve at each standard deviation
68% of the population fall between one standard deviation and 2 standard deviations etc….

10. ?

11. Imagine it is a whale!
Whale swimming towards vertical axis – coming home positive 
Away from vertical axis – leaving home negative 

12. ?

13. ?

14. ?

15. ?

17. H/W Task
1. Work out the mean, median and mode for each set of scores (3 marks)
2. Is the set of scores for the ‘sound alike’ condition normally distributed? Explain your answer (2 marks)

18. Further Practice QS – Revision
Contact me for answers to check your work

19. Normal distribution, Negatively or positively skewed scores?
Mean=?
Median=?
Mode=?
2. Provide a title
Frequency for the………

20. Normal distribution, Negatively or positively skewed scores?
Mean=?
Median=?
Mode=?
2. Provide a title
Frequency for the………

21. Indicate the position of the mean median and mode
Normal distribution, Negatively or positively skewed scores?
Scores 2 3 4 5 6 9 11 12 13
Scores 2 3 4 5 6 7 8 12 16
Scores 2 3 5 6 9 11 13 16