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• 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

p < (or equal to) 0.05

There is a 5% possibility that the results occurred by chance

The level of significance

P can vary e.g.1% for drug trials

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• TEST STATISTIC – single number

produced after doing particular set of calculations depending on test used e.g rho (Spearman’ correlation test) and U (Mann- Whitney)

• OBSERVED VALUE – Value of the test statistic

• 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)

Some tests are significant if the observed value is equal

to or exceeds the critical value e.g. Spearman’s and

Chi-square.For Mann-Whitney and

Wilcoxon to be significant the observed value should

be less than the critical value

Observed and Critical ValuesTo 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 DiscussionWrite 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?• Suggested Modifications?• Implications of research on society/psychology?• What could you do next to develop this research?

Look at an example -p 294-299 of old

Complete Companion A2

AbstractThis 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 study-

Inferential & 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