Methods

The DMHDS is a longitudinal study of the health, development, and behaviour of a cohort of 1037 young New Zealanders.⁷ The members of this cohort were born at the only obstetric hospital in Dunedin, New Zealand between 1 April 1972 and 31 March 1973. They have been followed up regularly since birth, with assessments every two years up to 15 years of age, and then again at ages 18 and 21 years. The follow up rates have been consistently high (approximately 95%).

Since its inception, one of the main components of the DMHDS has been a study of injuries and their prevention. An important aspect of this has been obtaining personal injury and crash records from study members. At each assessment information has been requested regarding injuries experienced since the previous assessment. The self reported injuries and crashes included in the present study were the outcomes of interest for a much larger longitudinal study of predictors of crashes and injuries among the cohort.⁸ The period of recall was three years and among the outcomes sought were: motor vehicle traffic crashes, where someone sustained an injury for which medical attention was sought; and injuries that required an overnight stay in hospital. This information was obtained in a face-to-face interview, by a trained interviewer, using a structured questionnaire. As an aid to recall, prompt cards listing injury and crash definitions were used.

In New Zealand most acute injury is initially treated in public hospitals.⁹ All discharges (hereafter referred to as admissions) from these hospitals are recorded in a database maintained by the New Zealand Health Information Service (NZHIS). All injuries are coded according to the International Classification of Diseases injury and poisoning codes.¹⁰ This database, therefore, provides a record against which the self reported hospitalised injuries can be compared. Signed consent was sought from each of the study members to access their hospitalisation injury records. All but one gave this consent. Using a unique individual identification number, the NZHIS file was searched to obtain the relevant records. The details on these records were compared (by the first author) to the self reported injury forms to identify matched records, and determine the completeness of self reporting.

For traffic crashes there is no database equivalent to NZHIS file. Although all crashes involving injury are required to be reported to the police, it is known that there is significant under-reporting of these events. For example, for crashes involving hospitalised injury the reporting rate ranges from 54% (motor vehicle passenger) to 70% (motor vehicle driver).¹¹ Conversely, some crashes involving minor untreated injury are included.¹² While this file is not suitable to determine the completeness of self reports, for the crashes that are recorded it includes many details that can be used as a reference to assess the accuracy of self reported details.

The traffic crash report file did not include a unique identifying number that could be used for the purpose of matching, so this was done using Automatch.¹¹ Automatch is a probabilistic and deterministic record linkage that has been used in a variety of settings including linkage of traffic crash records and public health databases. The details on the traffic crash report files identified as belonging to study members were compared with the details on self report crash forms to assess the accuracy of self report. These details included: vehicle type, year of crash, road user status, day of week, time of day, speed limit.

Results

A search of the NZHIS file revealed 45 injury related public hospital admissions for the participants in this study, over the relevant time period. The number and percentage of these records matched to the self report are given in table 1. The percentage self reported ranged from 86% (unintentional) to 14% (self inflicted)

A search of the police traffic crash records revealed 51 crashes that involved a member of the DMHDS cohort, over the relevant period of time. Of these, 25 (49%) were matched to self reported crashes. A comparison of the details recorded in the traffic crash report file with those self reported is summarised in table 2. These results show that the agreement ranged from 100% for type of vehicle to 76% for time of day (within one hour).

Discussion

The results of this study give us confidence that the data relating to unintentional injuries and motor vehicle crashes obtained by self report from the members of the DMHDS cohort are of good quality. Although the number of events examined was quite small, two aspects were especially encouraging. First, the reporting rate for unintentional injuries was high (86%). Second, the level of agreement between the details on traffic crash reports compared with the self reports suggest that self reports may be a valid source of crash information, without the biases associated with police reporting.

Although the numbers were very small, the level of self reporting of self inflicted injury was disappointing. Of the six unreported cases, however, five were drug overdoses. It is possible, therefore, that the study members involved did not consider these to be injuries. Alternatively, given the sensitive nature of these events, they may have chosen not to disclose them in the context of this interview.

Two points to be considered when interpreting the results of this study. First, because of the longitudinal nature of the DMHDS, the participants in this study may have developed better recall and are more willing to report socially undesirable behaviours than others who have not had this experience. Second, the procedure used to match the records may have lead to higher correlations than if this had been done “blind”.