Chapter 7: Analyses of Differences between Two Conditions: The t-test

Degrees of Freedom (DF)

Degrees of Freedom (DF)

________ should always be reported in your laboratory reports or projects, along with the t- value, p- value and confidence limits for the difference between means.

null hypothesis

This is the probability of your obtained t- value having arisen by sampling variation, or error, given that the ________ is true.

Degrees of freedom

________ are usually reported in brackets, as follows: t (87)= 0.78 This means that the t- value was 0.78, and the ________ 87.

Effect size

________: this is a measure of the degree to which differences in a dependent variable are attributed to the independent variable.

p value

The ________ shows you the likelihood of this arising by sampling error.

Gossett

________ worked for Guinness, whose scientists were not allowed to publish results of their scientific work, so ________ published results using his new test under the name of Student, which is why you will see it referred to in statistical books as Students t.

Inferential tests

________: t- tests discover how likely it is that the difference between the conditions could be attributable to sampling error, assuming the null hypothesis to be true.

confidence limits

SPSS, using the t- test procedure, gives you ________ for the difference between the means.2 The difference between means, for your sample, is a point estimate.

t test

The ________ assesses whether there is a statistically significant difference between the means of the two conditions.

Effect size

this is a measure of the degree to which differences in a dependent variable are attributed to the independent variable

Inferential tests

t-tests discover how likely it is that the difference between the conditions could be attributable to sampling error, assuming the null hypothesis to be true

Our estimate could be slightly different from the real population mean difference

thus it would be better, instead of giving a point estimate, to give a range

The confidence limits let you know how confident you are that the population mean is within a certain interval

that is, it s an interval estimate for the population (not jut your sample)

7.3

13.8 = -6.5

d = (x1

x2)/ (mean SD)

This means that we take one mean away from the other (it does not matter which is which

ignore the sign) and divide it by the mean standard deviation

Step 1

find mean sample SD

Step 2

find d

This means that your obtained t is under an area of the curve that is uncommon

by chance, you would not expect your obtained t-value to fall in this area

Degrees of freedom are usually reported in brackets, as follows

t (87) = 0.78 This means that the t-value was 0.78, and the degrees of freedom 87

The t-test is a parametric test, which means that certain conditions about the distribution of the data need to be in force

that is, data should be drawn from a normally distributed population of scores

This is because each participant performs both conditions, and so each participant can be tested against him

or herself

Related t-test gives a result with a higher associated probability value

• this is because the comparison of participants with themselves gives rise to a reduced within-participants variance, leading to a larger value of t