Statistical Methods for Psychology

The difference between recall with high levels of processing and recall with low levels of

processing is about two standard deviations, which is a considerable difference. Thinking

about the material you are studying certainly helps you to recall it.

Now suppose that you wanted to look at the effects of Age. Because we can guess that

people vary in the levels of processing that they normally bring to a memory task, then we

should add the main effect of Condition and its interaction with Age to the error term in

calculating the effect size. Thus

Because we only have two ages, the contrast ( ) is just the difference between the two

means, which is (13.16 2 10.06) 5 3.10.

In this case younger subjects differ from older participants by nearly two-thirds of a stan-

dard deviation.

Simple Effects

The effect sizes for simple effects are calculated in ways directly derived from the way we

calculate main effects. The error term in these calculations is the same error term as that

used for the corresponding main effect. Thus for the simple effect of Age for highest level

of processing (Imagery) is

Similarly, for the contrast of low levels of processing versus high levels among young par-

ticipants we would have

and the effect size is

which means that for younger participants there is nearly a 2^2 ⁄ 3 standard deviation differ-

ence in recall between the high and low levels of processing.

13.10 Reporting the Results

We have carried out a number of calculations to make various points, and I would certainly

not report all of them when writing up the results. What follows is the basic information

that I think needs to be presented.

In an investigation of the effects of different levels of information processing on the

retention of verbal material, participants were instructed to process verbal material in

dN=

cN

sN

=

9.15

3.48

=2.63

c=a 2

1

2

b(6.5) 1 a 2

1

2

b(7.6) 1 a

1

2

b(14.8) 1 a

1

2

b(17.6) 1 (0)(19.3)=9.15

dN=

°N

sN

=

(17.6 2 13.4)

4.98

=

4.20

4.98

=0.84

dN=

°N

sN

=

3.10

4.98

=0.62

°

=

B

2427.54

98

= 1 24.77=4.98

serror=

B

SSerror 1 SSCondition 1 SSA 3 C

dferror 1 dfCondition 1 dfA 3 C

=

B

722.30 1 1514.94 1 190.30

901414

Section 13.10 Reporting the Results 443