probability and significance

Question 1

our experiment

Answer 1

Investigating the impact of CBT on depression.
Hypothesis: There will be a difference in 25 participants’ depression scores on the BDI before and after a 6 week course of CBT.

Question 2

hypothesis-null vs alternate

Answer 2

For every study there are two types of hypotheses.
• The original / alternate hypothesis (H1) - there will be an effect.
• There will be a difference in participants’ depression score on the BDI before and after a 6 week course of
Свт.

The null hypothesis (Họ) - which states there will not be an effect
• There will be no difference in participants’ depression score on the BDI before and after a 6 week course of
СВТ.

A statistical test allows us to decide which of the hypotheses is true and this whether or we accept or reject Ho

Question 3

significance

Answer 3

Lets assumes for a minute we find a difference in depression scores before and after CBT.
• Turns out everyone improved.
• BUT - it doesn’t doesn’t mean the results are of value; that they are significant.
• Could have quite easily been down to chance, fluke or coincidence.
• → We need to use a statistical test.

Question 4

probability

Answer 4

All studies employ a significance level in order to check for significant differences or relationships.
• The accepted level of probability is 0.05 (5%) - this is the level at which the hypothesis is accepted.
• If the hypothesis is accepted, you are saying there is a less than 5% probability the results occurred by chance.
• There is a 95% probability that the results occurred due to a manipulation of the IV.

Question 5

choosing the wrong significance levels

Answer 5

type 1 error-If a too lenient significance level is used, e.g. 10%. Results in rejecting the null hypothesis that is in fact true.
Null Hyp = True
Alt Hyp = Wrong

type 2 error-If a too stringent significance level is used, e.g. 1%. Results in accepting a false null hypothesis.
Null Hyp = Wrong
Alt Hyp = True

Question 6

establishing significance

Answer 6

Working out whether or not results are significant requires being able to read the table on the right.
You will need several pieces of information.
1. The N value (how many participants)
2. The calculated value (S, M, T, U etc. - depends on the test)
3. Significance level (always 0.05 unless told otherwise)
4. Whether the test is one tailed or two tailed.
i. Directional or non-directional hypothesis.
5. The critical value.
Most of this information will be given to you