Validity of a questionnaire survey: the role of non-response and incorrect answers.

Errors in questionnaire surveys are usually of one of two sources: non-responses or incorrect answers. The aim was to investigate the validity of a questionnaire survey and to estimate the respective bias of these answers. Of 9,283 subjects selected to receive a questionnaire by post, 3,949 (43%) responded, and, of these, 3,400 correctly reported their Swedish social security number. Answers in the questionnaire survey were given as proportions of the claims registered at local insurance offices. In the group of respondents who had correctly reported their social security number, the answers were compared individually with the registrations in dental insurance claims. In Sweden, these claims are labeled with the patient's social security number and it is thereby possible to make such comparisons. It was shown that errors were caused by non-response and also by respondents giving incorrect answers. Incorrect answers accounted for approximately one-third of the total bias. The remaining bias was caused by a non-response error. It is concluded that questionnaire studies have a bias caused by both non-response and incorrect answers and that together these can be substantial. Scientific reports that include questionnaire surveys must describe the procedure carefully. If possible, other sources of information should be considered.

Pattern of attendance to dentists and hygienists in the County of Göteborgs and Bohus län during 1990-1992.

AIM: To describe the dental care consumption in a random sample of the population of a Swedish county 1990-1992 and to compare this with results from other Swedish studies, especially those using different self-reporting methods. SUBJECTS: One sixtieth of all adult inhabitants in the County of Göteborgs and Bohus län (all individuals born on the 17th day of an odd month) formed the sample. DESIGN: All insurance claims sent from both public and private dentists to the local insurance offices were registered in a data base. Thus every treatment registration was collected into this data base, independently, whether it was made in a private clinic or in the Public Dental Service and whether or not by a dental hygienist or other grade. Since almost all dental care in Sweden was performed within the dental insurance system, these registrations could be regarded as very near to the total dental care consumption of this group. The registrations were related to the age of the patient, the type of care (acute/regular), regularity, dental hygienist treatment and other factors. RESULTS: It was found that the youngest and the oldest patients visited a dentist less often, that approximately a quarter received hygienist care, that many received emergency treatment sometime during the observation period but that a majority also had regular dental care. Studies using a self-reporting technique reported a systematically higher level of consumption than those using insurance claim registration. CONCLUSIONS: Dental care consumption studies using self-reporting methods will probably overestimate the actual consumption. Emergency treatment is frequent even among patients who go to a dentist regularly.

Prediction of caries activity in children with today's low caries incidence.

One hundred 14-yr-old children were observed over 1 yr to find out if caries incidence and caries progression could be predicted in a low prevalence child population by means of well-known caries related factors. The mean caries incidence was low (0.45, SD 0.70) but, on the other hand, 32% of the children developed at least one new lesion during the test period. In only eight out of 35 children progressing lesions were demonstrated. Independent variables at baseline examination were caries prevalence, sucrose intake, fluoride exposure, oral hygiene, saliva secretion rate, and salivary concentrations of mutans streptococci and lactobacilli. A weak but statistically significant correlation was demonstrated between caries incidence and caries prevalence. No other significant correlations were shown. It was concluded that caries activity could not be predicted in this population. Low disease prevalence was a major reason for the weak correlations.