Lesson 15: Statistical Testing – The Sign test (When to use the sign test and calculations) Flashcards
What are the 2 types of statistics
a) Descriptive statistics (such as measures of central tendency and dispersion)
b) Inferential statistics.
What are levels of significance
The level at which the decision is made to reject the null hypothesis in favour of the experimental hypothesis. It states how sure we can be that the IV is having an effect on the DV and this is not due to chance.”
The level at which the decision is made to reject the null hypothesis in favour of the experimental hypothesis. It states how sure we can be that the IV is having an effect on the DV and this is not due to chance.”
What is probability
Probability is a numerical measure that determines whether our results are due to chance or whether there is a real difference that exists between the experimental and control conditions (and therefore we can accept the experimental hypothesis). If a real difference exists (that can be calculated statistically) we can say that results are significant, and the null hypothesis can be rejected (and we would accept the experimental hypothesis)
What significance levrks are used in psychology
The conventional and standard level of significance is expressed as:
p<0.05 (5% level). The 5% level of significance is mainly used in Psychology because:
A) It is not too strict or too lenient, but is a middle, fair value of significance
B) It minimises the chances of making a Type 1 or a Type 2 error (we will revisit this at A2 level).
•“p” stands for “probability” and the 0.05 (5%) value illustrates the level of significance that has been chosen (5% level of probability that results are due to chance/fluke, therefore 95% certainty that our results are showing a real difference between control and experimental conditions)
•This means that if the level of significance is achieved, then the probability of results being due to chance (or fluke) is 5% or less. This can also be expressed as p = 0.05, but we tend to use p<0.05 (5%)
•5% significance levels are usually used when there is a directional one tailed hypothesis that has been clearly stated in the research
•Sometimes a 10% level of significance is selected, and this is expressed as; p<0.10 (10%), and this is often used when we allow a 10% margin of error, and we would be 90% certain that our results are really showing a significant difference
•Sometimes a very strict level of significance is selected at 1% which is expressed as: p<0.01 (which indicates there is a 1% probability that the results are due to chance). This is often used when research findings are critical and are very important e.g. when testing the effect of drugs on humans, we must make sure that results are not due to fluke but that a real difference occurs between the experimental and control conditions, and that is why we set a stricter significance level.
When to use a sign test
1) We are looking for a difference between data, e.g. the drug HP10 makes people happy or not so happy
2) We are looking at paired or related data. The two related pieces of information could come from a repeated measures design or a matched pairs design because the participants are paired for the purposes of statistics, as one person tested twice.
3) The data is nominal (placed in categories, e.g. people are either happier or not happier once they have taken the drug HP10)
Steps of a sign test
State hypothesis - one or two tailed
Record data - work out sign + or -
Find S value - the less frequent sign number
Find the critics value
What is N
N is the total number of participants (ignoring any participants that gained a change of 0, e.g. no positive or negative score overall
How do you find the critical value
The hypothesis we stated earlier is two tailed non directional and therefore a two tailed test is used.
Now we use the table of critical values (below) and locate the 0.05 column for a two tailed non directional test and the row that begins with our N value (12).
For a two tailed test at p<0.05, we need to compare our S value to the critical value.
The calculated value of S must be EQUAL TO or LESS THAN the critical value for significance to be shown
