statistical testing

Question 1

explain the concept of significance

Answer 1

just because we found a difference in the mean number of words spoken in the two conditions, we do not know if this is what psychologists refer to ass a significant difference

The difference found may have been no more than that which could have occurred by chance, that is, by coincidence or fluke

Question 2

how do we find out whether a difference is significant

Answer 2

to find out whether a test is significant, we need to use a statistical test

Question 3

what is a statistical test

Answer 3

statistical tests provide a way of determining whether hypotheses should be accepted or rejected. In psychology, they tell us whether difference or relationships between variables are statistically significant or have occurred by chance

Question 4

what is an example of a statistical test

Answer 4

one statistical test is a sign test

a sign test is a statistical test that is used to analyse the differences in scores between related items (e.g. some participants are tested twice) the data should be nominal or better

Question 5

what conditions are required in order for a sign test to be carried out

Answer 5

to use the sign test:

1. we need to be looking for a difference rather than an association
2. we need to have used a repeated measures design
3. we need data that is organised into categories known as nominal data - if the data is not nominal, we can convert it for the purposes of this test

Question 6

what is nominal data

Answer 6

nominal data is defined as data that is used for naming or labeling variables

examples of nominal data: gender (male, female) favourite colour (red,green, blue)

Question 7

what is the accepted level of probability in psychology / the significant level

Answer 7

all studied employ a significant level in order to check for significant differences or relationship

the accepted level of probability in psychology is 0.05 5%

this is the level at which the researcher decides to accept the researcher hypothesis or not

Question 8

what does it mean when the experimental hypothesis is accepted

Answer 8

if the experimental hypothesis is accepted, this means there is less than 5% probability that the results occurred by chance

In simple terms, this means the researcher can be pretty certain that the difference found was because of the manipulation of the independent variable (though there will always be 5% doubt even if significance is found)

Question 9

why do we sometimes employ a more stricter significance level

Answer 9

in some circumstances, researchers need to be even more confident that results were not due to chance and so employ a stricter, more stringent significance level such as 0.01 (1% level)

this case may involve human cost e.g. new drugs are being trialed, or when a particular investigation is one off and there is no possibility that it can be repeated in the future

Question 10

what is the calculated value

Answer 10

when the statistical test has been calculated, the researcher is left with a number - the calculated value

this number is compared with a critical value to decide whether the result is significant or not

Question 11

how do we calculate the critical values in a sign test

Answer 11

the critical values for a sign test are given on a table of critical values

You need the following info to use the table:

1) the significance level desired ( this is always 0.05 or 5% except in the cases described previously)
2) thee number of participants in the investigation ( the N value)
3) whether the hypothesis is directional (one - tailed) or non - directional (two tailed)

Question 12

how can we use the calculated critical values to tell if the data is significant

Answer 12

if the calculated value is equal to or lower than the critical value for the result to be regarded as significant

Question 13

the sign test a worked example

Answer 13

Take the data we collected form the energy drink experiment to calculate the sign test

STEP 1:
covert the data to nominal data by working out which participants produced a higher work count after the energy drink, and which produced a lower word count

we do this by subtracting the score for water from the score for SpeedUpp
If the score is negative we simply record this, if the answer is positive we record a plus sign

STEP 2:
we add up the pluses and minuses from the table
The number of pluses (13) is the number of participants who spoke more words in the 5 mins often drinking SpeedUpp than they did after drinking water
The total number of minuses (T) is the number of participants who spoke more words in the five minutes after 5 mins after drinking water than they did after drinking SpeedUpp

STEP 3:
we take the less frequent sign (in this case it is the total number of minuses) and call this S. Therefore S = 7 (S is our calculated value)

STEP 4: 
now we can compare our  our calculated value with the critical value 
-hypothesis is directional 
-0.05 significance
-N=20  

the calculated value value 7 is more than the critical value of 5
Therefore, the difference is not significant at 0.005 level