Transcript blog using inf stats dec 09

Inferential Stats, Discussions and
Abstracts!!
BATs
Identify which inferential test to use for your
experiment
Use the inferential test to decide if your
hypothesis can be accepted or rejected
Write a discussion based on your statistics
and relate to background research
Write an Abstract for your piece of research
Inferential Statistics
• Allow psychologists to make inferences
about the findings of a piece of research
when only testing a sample of the world’s
population.
• Permit psychologists to work out, for a
given probability, whether a pattern in their
data has arisen by CHANCE or because
there was a real difference/correlation –
i.e. the pattern is SIGNIFICANT.
Inferential Statistics
• In order to use an Inferential test you need
• A null hypothesis
• An alternative hypothesis
Inferential Statistics
P can vary
e.g.1% for
drug trials
• What do we mean by chance?
• Psychologists decide on a probability that
they will ‘risk’. They state how certain
they are that the findings were not due to
chance.
Psychologists use a PROBABILITY of
The level of
significance
p < (or equal to) 0.05
There is a 5% possibility that the results
occurred by chance
Type 1 and Type 2 Errors
A 5% significance level is used because..
• The degree of uncertainty is not a life or death
matter (medical research uses 1%)
• If the level of significance is too high (lenient)
e.g. 10% you may reject a null hypothesis that is
actually true. TYPE 1 ERROR
• If level of significance too low (stringent) e.g 1%
you may accept the null hypothesis when it is
not true. TYPE 2 ERROR
Levels of Measurement
• Before deciding which test to use you need to
identify the LEVEL of MEASUREMENT used.
• NOMINAL – data in separate categories e.g.
red, blue and green
• ORDINAL – data ordered e,g putting people in
order of height. The ‘difference’ between each
item is not the same
• INTERVAL – Data using units of equal intervals
e.g. counting number of correct answers
• RATIO – There is a true zero point e.g. time
taken, distance travelled e.t.c
Observed and Critical Values
•
•
Some tests are significant if
TEST STATISTICthe
– single
observednumber
value is equal
produced after doing
set
of
to orparticular
exceeds the
critical
calculations depending
onSpearman’s
test usedand
e.g
value e.g.
Chi-square.
rho (Spearman’ correlation
test) and U
(Mann- Whitney) For Mann-Whitney and
Wilcoxon to be significant
OBSERVED VALUE
– Valuevalue
of the
test
the observed
should
be less than the critical
statistic
value
• CRITICAL VALUE – value the test stat
must reach in order for the null hypothesis
to be rejected (these are looked up in a
special table for that test)
Observed and Critical Values
To find the appropriate Critical Value in a
table you need to know …
• Degrees of freedom (df) – relates to the
number of ppts in the study (N).
Independent groups have 2 values of N –
N1 and N2. In Chi Square the df relates to
the number of cells used.
• One-tailed or two-tailed test – directional
hypothesis = one tailed, non-directional =
two tailed
• Significance level – p < (or equal to) 0.05
Which Inferential Statistic?
• Use the text book p286-296 to help you
decide which Inferential test is most
appropriate for your research
• Note down what test you will use and why!
• Now try the test out on your data
• Was your data significant?
• Can you accept or reject your Hypothesis?
Writing a Discussion
Write a discussion of your findings. Include the
following …
• What did you find out? – specifically mention your
descriptive stats – do they match your hypothesis,
were they significant – mention the Inferential test
stats
• Do your findings match the original research?
• Limitations?
Look at an example -p
294-299 of old
• Suggested Modifications?
Complete Companion
A2
• Implications of research on society/psychology?
• What could you do next to develop this research?
Abstract
This is basically a summary at the start of a report to ‘whet the reader’s
appetite’!!
• No more than 250 words long, it should concisely summarise
the…
1. Aim- The aim was to investigate……
2. Background-Your hypothesis (null and alternative).
3. Design- Independent/Repeated Measures.
Variables.
4. Sample- Selection? Girls? Boys.
5. Results and statistical conclusions of your studyInferential & Descriptive..Main findings.
• It should be written in the 3rd person and past
tense, in other words you should not write ‘I
found….’
instead ‘The experiment was carried out …
Mock next week – Thursday!!
• Revise all you have done on Addictive
Behaviour and Research Methods