Design

The study took place in two phases: a memory generation phase (phase 1), and an induced-forgetting phase (phase 2), each of which is described in the procedure section. In the memory generation phase, a quasi-experimental

between-subjects design in which the between-subject variable was mood with 2 levels (non-dysphoric and dysphoric) was used to examine group differences on several variables. First, dysphoria was measured and subsequent scores used to categorize individuals as either dysphoric or non-dysphoric. Three variables

measuring mood state were examined (sadness, positive, and negative affect).

Sadness, positive, and negative affect were measured before memory generation and after memory generation to determine the effect of the memory generation task on aspects of mood state. Generation time (the time to generate a

predetermined set of 32 negative memories) was also measured.

Finally, 3 variables were measured to ensure that groups did not differ in terms of how negative, clear, and old their memories were. These same variables were then used to provide a means of methodological control over memory characteristics among each induced-forgetting paradigm. That is, for the DF paradigm all memories were distributed among designated be-forgotten or to-be-remembered categories such that clarity, negativity, and age ratings were equal between the two categories. For the RIF paradigm, the same procedure was used to distribute memories equally according to negativity, clarity, and age among the Rp+, Rp-, and NRP categories of the paradigm. This was designed to ensure that differences in paradigm efficacy could be attributed to paradigmatic differences and not differences in memory characteristics.

The induced-forgetting phase of the present study was designed as a 2 (condition) by 2 (mood) by 3 (forgetting paradigm) mixed factorial design (see Table 1 below). Using prior research as a guide (e.g., Barnier et al. 2004; Joslyn et al., 2005), it was decided that a sample of 20 participants per cell should be adequate to demonstrate the forgetting of autobiographic events. Given the moderate to large size of the effects obtain in previous studies, and typical cell

sizes of 15 to 25 participants, 20 participants per cell was deemed adequate to demonstrate an effect if one existed. Using a mixed factorial design in which there was one within-subjects variable, a total sample of 120 participants were required.

Table 1. Diagrammatic representation of the research design for Phase 2.

Paradigm

Mood Instruction DF-Item DF-List RIF Non Dysphoric Exp. Condition

Baseline Condition

For the purpose of demonstrating a forgetting effect, there was 1 within-subject variable (Instruction) in which participants served as their own controls with 2 levels: experimental (E) and baseline (B) condition. The baseline condition was the proportion of words remembered when told to remember (RR) for DF or the unpracticed unrelated list of memories in RIF (denoted as NRP). The

experimental condition was the number of words remembered when told to forget (RF) or the related but unpracticed list of memories in RIF (denoted by Rp- for RIF). There were also two between-subjects independent variables: mood state with 2 levels: non-dysphoric and dysphoric. These groups were formed using a second CES-D scale administered during this second phase following cut-off scores of lower than 16 to indicate non-dysphoric and 16 or higher to

define the dysphoric group; and forgetting paradigm with 3 levels: directed forgetting–item (DF-item) method, directed forgetting–list (DF-List) method, and retrieval induced-forgetting (RIF) method. The dependent variable was the total number of memories remembered.

Additional variables measuring various aspects of mental control including: perceived mental control, different mental control strategies (e.g., distraction, punishment, worry, social control, and appraisal), and rumination on sadness, were examined in terms of the relationship among these aspects of mental control and memory, including, accessibility (operationalised as the time to generate), directed-forgetting effect (RR-FF) in the case of DF, and degree of impairment (NRP minus Rp-) in the case of RIF.

2.2.1 Analytic Strategy

According to Tabachnick and Fidell (2007), a meaningful regression

solution can only be achieved with an adequate sample size. The authors suggest that in order to test the multiple correlations, a sample size > 50 + 8m (where m is the number of predictors) is required. Further, to test the individual predictors, Tabachnik and Fidell also indicate that the sample size should be >104 + m. For the present study, there were 10 predictor variables. In order to test the multiple correlations, sample size of more than 130 would be deemed sufficient to provide meaningful correlations and a sample size of 114 is adequate to test the

individual predictors. The authors also advise that a more stringent sample size calculation be used when anticipating anything other than large effects.

Therefore, the present study utilized the mixed factorial ANOVA, t-tests with a Bonferoni correction, and Pearson correlations to evaluate hypotheses. Although an ANOVA approach is not without its constraints, one of which is using

continuous variables in a dichotomous fashion, the ANOVA adequately addresses the questions of group differences and is deemed an appropriate analytic strategy for this study.

Finally, while not the best methodological strategy, it is common practice within research in medicine, psychology, and other behavioural sciences

(Altman, 2006) to construct groups based on a single continuous measure of demographic and psychological construct (i.e., age, anxiety). This is often done at the expense of power and the present study acknowledges this cost.