Lesson 6

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Lesson 6
Lesson 6
NORMAL AND SKEWED DISTRIBUTION
TYPE ONE AND TYPE TWO ERRORS
Level of significance and errors

Level of significance accepted in psychology?

0.05 - 5%

0.1 - 10% 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?
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
Normal and Skewed Distribution

Small samples – central tendency and standard deviation  useful
statistics

Larger samples – useful to examine overall distribution formed by the
data
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
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

Distribution – only considered for interval data

So that mathematical calculations can be carried out
Normal distribution
Positively skewed
distribution
Negatively skewed
distribution
When scores are
clustered around
the mean and
when the mean
median and mode
are similar
When scores are
clustered below
the mean and
the mode shows
that
When scores are
clustered above
the mean and the
mode shows that
Normal
distribution,
Negatively or
positively
skewed
scores?

Mean=?

Median=?

Mode=?

2. Provide a
title

Frequency for
the………

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….
?

Imagine it is a whale!

Whale swimming towards vertical axis – coming home positive 

Away from vertical axis – leaving home negative 
?
?
?
?

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)
Further Practice QS – Revision

Contact me for answers to check your work
Normal
distribution,
Negatively or
positively
skewed scores?

Mean=?

Median=?

Mode=?

2. Provide a
title

Frequency for
the………
Normal
distribution,
Negatively or
positively
skewed scores?
Mean=?
Median=?
Mode=?
2. Provide a title
Frequency for the………
Indicate the position of the mean
median and mode Normal distribution, Negatively or
positively skewed scores?
Scores
Scores
Scores
2
2
2
3
3
3
3
4
5
3
5
6
4
6
6
5
7
9
6
8
9
6
8
9
9
12
11
11
12
11
12
12
13
13
16
16

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