H) Event knowledge with each interview

Given that the interviewers in this study were naive about the details of the event until the first interview, it is important to check what impact each successive interview had on amount of information gained from witnesses. For each interviewer in each condition, interview position (first interview, second) was correlated with amount of correct information, errors and confabulated details. There were no significant correlations.

Analysis of interview techniques

On the whole, the interviewers used the techniques that were expected of them with all the CI interviewers using the `transfer of control' and `report everything' instruction. Similarly, they all used context reinstatement for the free report and all used CI probes in the questioning phase.

The SI interviewers did not on the whole use the CI instructions except for the `report everything' instruction which was used four times (17%). In accordance with instructions SI interviewers used the `transfer of control' in most (74%) of their interviews but less frequently than did the CI interviewers who used the instruction on every occasion (Chi-square(2) = 47.65, p<.001). The differential use of this instruction in the SI group was due to one of the interviewers who did not transfer control in 60% of her interviews.

The untrained interviewers did not use either the CI or SI techniques with the exception of one interviewer who used the instruction to `report everything' on a couple of occasions.

Number of questions asked

The mean number of questions asked in the CI, SI and UI were 17.73, 15.47 and 7.41 respectively (F (2, 63)= 6.21, p<.01). Post-hoc analyses showed significantly more questions were asked in CI than UI (Fisher PLSD= 4.34, p<.01) and in SI than UI (Fisher PLSD= 4.29, p<.05). There were no significant differences between CI and SI in the number of questions asked. The number of leading and misleading questions was less than 1%.

Interview length

The CI interviews were longer than the SI interviews and CI and SI were in turn longer than the untrained interviewers. This is what we typically find with the enhanced CI and is expected for the following reasons. The CI procedure takes longer to administer, the instructions are more complex, the witness takes longer to respond and most importantly the witness says more. Since the aim of a CI is to elicit more information, it would be natural for the interviews to take longer.

Discussion

The major aim of this paper was to compare the effects of the Cognitive Interview with an interview procedure (SI) as similar as possible to the CI but not containing the special CI techniques of context reinstatement, imagery and `report everything.' When this was done there was no significant advantage of the CI over this type of SI suggesting that increases in amount reported in the many earlier studies CI may be a consequence increased rapport with the witness rather than the use of cognitive CI techniques.

The similarity in performance of cognitive and structured interviewers is interesting and important as it suggests that when rapport and communication are improved significant gains in information may be achieved. However, the role of the cognitive retrieval aids cannot be ruled out. The means consistently favoured the cognitive over the structured, the data are nevertheless inconsistent with the results of some recent studies (using adults as witnesses) which did find the cognitive group significantly superior to the structured group (Koehnken et al, 1994b; Koehnken et al, 1995; Mantwill et al, 1995). It has been suggested that perhaps our cognitive and structured groups more closely resembled each other than those in previous studies (Fisher, personal communication). It may be the case that the communication and cognitive elements of the CI work in tandem. For example, in order to effectively use the context reinstatement strategy, the witness needs time to concentrate and respond without interruptions from the interviewer. Along similar lines Fisher and McCauley (1995) have suggested that the effects of CI reflect both improved memory search and improved communication (see also Memon & Stevenage, 1996). This provides an illustration of the difficulties of studying memory in real world contexts. An interview is an interaction between two people and memory performance is thus undoubtedly influenced both by the technique used to search memory and rapport with the person who is guiding the retrieval process (see Memon, Wark, Holley, Bull and Koehnken, 1996c for a discussion of the importance of interviewer behaviour). We feel this has brought us to the end of our search for what underlies the success of the cognitive interview. If we were to breakdown the CI (and SI) further to examine what was going on, it would mean sacrificing ecological validity. On the other hand, there is much that remains to be understood about the mechanisms underlying CI effects and the conditions under which the CI may be most effective. It is to this that our discussion will now turn.

The CI and SI elicited more correct recall but also more errors and confabulated details than the UI. One plausible explanation for the increase in both correct and incorrect information with the CI and SI is that it was simply due to the greater number of questions asked by CI and SI interviewers. The CI and SI interviewers began their questioning with open-ended questions but probed for details with closed questions. In contrast, the untrained interviewers asked relatively few questions overall and fewer closed questions. One consequence of asking so many questions is that it places pressure on witnesses to give a response rather than providing them with the option of choosing what to report and withholding details about which they are uncertain (see Koriat & Goldsmith, 1994 for a discussion of the effects of cued versus free recall testing on accuracy). The fact that increases in gains in correct information were found for the same types of information (person details) as the errors suggests that a plausible explanation for the increase in errors is that the CI technique shifts the criterion for responding by influencing confidence and willingness to report information (the theoretical basis is illustrated by signal detection theory as noted by Bekerian and Dennett, 1993). Interestingly, a similar effect (an increase of both correct and incorrect) is sometimes seen with hypnosis and where this occurs it is interpreted in terms of a shift in response criterion (Orne, Soskis, Dinges & Orne, 1984). Krass, Kinoshita and McConkey (1989) argue that under hypnosis, subjects may offer as memories reports they would normally reject on the basis of uncertainty. Asking more questions also means generating more information overall and this may also explain why accuracy rates (the proportion of correctly recalled details) were slightly lower in CI/SI groups. The latter was particularly discouraging as the typical finding in our laboratory and others is that there are no differences in accuracy rates between the CI and SI (see Memon and Stevenage, 1996 for a recent review). An anonymous reviewer has suggested that perhaps the witnesses in the present study were less cautious because they were aware of the artificiality of the situation, they knew their testimony would not have any serious implications. The reviewer suggests that it may be possible to obtain more accurate recall by placing special emphasis on the importance of veridicality of reports. This could be done by repeatedly reminding witnesses not to guess or fabricate responses[5] or by asking for certainty judgements.

One of the aims of this study was to compare the CI (and SI) with an untrained group. It could be argued that our untrained group (university students) are not typical in that they tended to ask fewer questions than our trained group, did not interrupt the witness when they were speaking and did not ask leading questions. In other words, they did not possess the interviewing characteristics of police officers as described by Fisher et al. (1987) and George (1992). They also made fewer errors and confabulations. This could simply be due to the fact that our untrained interviewers were aware that their interviews were being recorded and therefore were cautious in their questioning strategy. This possibility has to be taken into consideration in interpreting our results. In terms of practical implications we cannot recommend that interviewers can effectively collect information without training because our untrained group may not be representative. However, our SI group may provide more information about the likely impact that training in basic communication and rapport building is likely to have. Indeed as practice improves and training is identified as an important issue in police organisations (Cherryman & Bull, 1992) the result is likely to be more effectively conducted interviews which yield more information.

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