Chapter 7: Analysing the Data
Part IV : Analysis of Variance

Scenario and Data Set ^#2

The following example is from Howell page 300, which in turn was based on Eysenck (1974).

Craik and Lockhart (1972) proposed as a model of memory that the degree to which the subject remembers verbal material is a function of the degree to which it was processed when it was initially presented. Thus, for example, if you are trying to memorise a list of words, repeating a word to yourself (a low level of processing) would not lead to good recall compared to thinking about the word and trying to form associations between that word and some other word.

Fifty subjects aged between 55 and 65 years were randomly assigned to one of five groups. The five groups included four incidental learning groups and one intentional learning group. Where incidental learning is learning in the absence of expectation that the material will later need to be recalled. The Counting group was asked to read through a list of words and simply count the number of letters in each word. This involved the lowest level of processing, since subjects did not need to deal with each word as anything more than a collection of letters.

The Rhyming group was asked to read each word and think of a word that rhymed with it. This task involved considering the sound of each word, but not its meaning. The Adjective group had to process the words to the extent of giving an adjective that could reasonably be used to modify each word on the list. The imagery group was instructed to try to form vivid images of each word. This was assumed to require the deepest level of processing of the four incidental conditions. None of these four groups were told that they would later be asked for recall of the items.

Finally, the Intentional group was told to read through the list and to memorise the words for later recall.

After subjects had gone through the list of 27 items three times, they were given a sheet of paper and asked to write down all the words they could remember. If learning involves nothing more than being exposed to the material then the five groups should show equal recall. If the level of processing of the material is important, then there should be noticeable differences among the group means. The data (number of words recalled) are presented below.

Research question

The research question is obviously whether the level of processing required when material is processed affects how much material is remembered.

Figure 7.2 Data for one-way between groups ANOVA.

Hypotheses:

H_o : µ ₁ = µ ₂ = µ ₃ = µ ₄ = µ ₅

H₁: µ s not all equal

Assumptions

Howell 302-303.

The between groups ANOVA requires the same assumptions as the between groups t-test. These are:

1. All observations must be independent of each other
2. The dependent variable must be measured on an interval or ratio scale.
3. The dependent variable must be normally distributed in the population (for each group being compared). (NORMALITY ASSUMPTION)
4. The distribution of the dependent variable for one of the groups being compared must have the same variance as the distribution for the other group being compared. (HOMOGENEITY OF VARIANCE ASSUMPTION

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