Low Trait Anxiety Group (LTA) (n = 20)

2.3 Selection And Recruitment Of Participants

General Inclusion and Exclusion Criteria for all Participants

All participants fell in the age range 18-65 years. Any potential participants who had significant medical problems were excluded from the study, since this would be likely to increase levels o f somatic self-focus. Also, because the tasks required verbal ability, potential participants were excluded if their command of English was judged by the researcher to be insufl&ciently good.

Selection of Patient Participants

Two groups o f clinical psychology outpatients were recruited:

1. Those fulfilling DSM IV diagnostic criteria for Panic Disorder (with or without Agoraphobia) and Major Depressive Disorder (FDD)

2. Those fulfilling DSM IV criteria for Major Depressive Disorder alone (D).

The diagnostic criteria are presented in Appendix 1. Subjects were excluded if they met the DSM IV criteria for any other Axis 1 or Axis 2 disorder. Fulfilment o f diagnostic criteria was assessed at the beginning o f the testing session by means o f a semi-structured interview (see Appendix 2.).

Recruitment of Patient Participants

Patient participants were drawn from four outpatient Clinical Psychology services. Details of the selection criteria described above were sent to clinicians working in these

services and any patients who were thought to fulfil the criteria were invited to participate in the study by the clinician who was treating them. Patients who agreed to participate were then contacted via letter by the researchers and invited to attend a single testing session at the relevant hospital. Testing took place either during the assessment period or early in treatment.

Selection of Healthy Controls

Control were allocated to either a High Trait Anxiety group (HTA) or Low Trait Anxiety group (LTA) on the basis o f their trait anxiety score on the Spielberger State- Trait Anxiety Inventory Form Y(Spielberger, Gorsuch, Lushene, Vagag, & Jacobs, 1983). Participants who scored within one standard deviation (i.e., a score of 45 or less) of the Spielberger norms were considered to have low trait anxiety. Those whose score exceeded one standard deviation from the norm (i.e., 46 or above) were allocated to the high trait anxiety group. Participants who had consulted their GP for any psychological problem during the previous year were excluded.

Recruitment of Controls

Controls were volunteers drawn fi*om the volunteer panel of a teaching hospital. Potential participants were contacted by telephone and invited to attend a single testing session at the hospital if they were willing to participate. As the hospital operated a fixed system of recompense to research participants recruited from the volunteer panel, the controls were paid the standard fee of £6 for their participation in the study.

Matching of Groups

In order to reduce error variance it was planned to match all four groups on three potentially confounding variables. These were average age, ratio of females to males within the group and educational level. For the latter, participants were assigned to one of three categories (no qualification, qualifcations up to ‘A’ level and graduate). Although the matching was not exact, statistically significant differences between the groups on these variables were not present - further information on this point is presented in the Results section.

2.4 Statistical Power

Whereas the methods of inferential statistics provide safeguards against Type I errors (i.e. false positives), it is also necessary to consider Type U errors (false negatives) by building adequate statistical power into the experimental design. Statistical power refers to the probabiUty that the study will detect an effect that is actually present. Carefijl choice of measures and control of error variables enhance the power of a study, but a further

important aspect is the use of a sufficiently large sample. The required sample size can be calculated with reference to the expected effect size and the level o f confidence required (Cohen,1992).

There has been only one previous study Borden et al. (1993) comparable to the present research and insufficient information was presented for a precise estimation o f the effect size to be made. However, Cohen defines a large effect as being one ‘likely to be visible to the naked eye of a careful observer’ (Cohen, 1992, p. 156). The results o f Borden et al.

(1993) fulfil this criterion and a large efifect size has therefore been assumed for the present study. In order to achieve a power of .80 (for alpha = .05), a sample o f 18 participants per group is required (Cohen, 1992). Thus, the statistical power o f the present study was considered to be adequate.

2.5 Ethical Procedures

Permission to carry out the study was obtained from the research ethics committees for each institution from which participants (both patients and controls) were recruited (copies of relevant documentation appear in Appendix 3 ). In order to ensure that participants could give informed consent, a participant information sheet outlining the nature o f the tasks (see Appendix 4. ) was sent to individuals prior to the testing session. All participants were given an opportunity to raise any concerns at the beginning o f the testing session before being asked to sign a consent form (see Appendix 5 ). In addition, a thorough debriefing was given after the tasks and any further action which was thought necessary (eg. liaison with the referring clinician) was undertaken at the request o f the participant after written consent had been obtained. In accordance with the requirements o f the ethics procedures, the GPs o f all patient participants were informed that the

2.6 Measures

The following section describes the measures used in the study, both for

selection/screening and for testing the experimental hypotheses. In order to retain clarity of presentation, information concerning the rehabihty and validity of the measures is detailed separately in Appendix 6.