M ETHODOLOGICAL ISSUES

This thesis is based on data collected in three cross-sectional studies. Stratified random sampling was used and data collected by oral clinical examination and administered questionnaires. Paper I is based on data collected by use of a reported quantitative questionnaire. Papers II and III used data collected by self-reported questionnaires and clinical examinations.

5.1.1 Subjects

The participants were selected in two ways. The participants in Paper I were randomly selected from the total population of 20 to 80 year old Norwegians. The sample was drawn from a stratified population in the national registry based on age, sex and place of residence. Within each stratum, a proportional random sample was drawn. The datasets are representative of the non-institutionalized adult population. The distribution of participants by demographic factors corresponded well with that of the Norwegian population. The response rate was 69%, evenly distributed among the age groups. The participants in the second study were based on samples from the birth cohort 1929 to 1938 and the sampling procedure had been examined in earlier years in Nord-Trøndelag. The sampling selected using the same procedure before and is regarded as representative of a Norwegian population. The response rate was 60.4%.

The third study was based on samples from the birth cohorts 1962 to 1971 and 1959 to 1960 and the sampling procedure had been examined in earlier years in Nord-Trøndelag. The response rate was 62%.

No statistically significant difference was found between those who participated and those who declined to participate with regard to age and sex in Studies II and III.

Obtaining high response rates usually lowers the probability of serious non-response bias (Lesaffre et al. 2009). A response rate between 60% and 69% must be regarded as

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acceptable today. The sampling procedure and the acceptable response rate indicate that the results may be regarded as representative for the Norwegian population.

5.1.2 Questionnaires - validity

An instrument is said to be valid if it measures what it purports to measure (McDowell and Newell 1996).

In all papers, questionnaires were used to assess self-rated oral health, dental visits, oral hygiene habits and demographic variables. To assess OHRQoL, the OHIP-14 was used in Papers I and II and the Dental Impact Profile in Paper III.

The OHIP-14 is a well known questionnaire, and has been found to be valid and reliable (Locker 1994, Slade 1997). It is widely used and, having been translated into several languages, it is consequently useful for comparisons between countries. In all papers, there was clear agreement between self-rated oral health and the scores on the instruments used to measure OHRQoL. Those who rated their oral health as good reported a better OHRQoL, which strengthens the validity of the questionnaires used to assess OHRQoL. In addition, there was agreement between number of teeth and OHRQoL a finding, which has been reported in many studies (Nutall et al. 2001, Steel et al. 2004, Locker, Gibson 2005, Åstrøm et al. 2006). This further strengthens the validity. The Dental Impact Profile has been used only once before but its development has been thoroughly described and was found to be valid and reliable (Strauss et al. 1993). The factor analysis in the present study resulted in a somewhat different pattern than the original subscales. The original subscales were constructed based on a factor analysis in an elderly population in North Carolina, USA and the authors hypothesized that the impact of teeth or dentures on a person’s life would be age dependent and reflect the different values and experiences of various cultural groups. It was also reported, in the development of the instrument, that most of the use of the instrument was based on the total score and not on subscales. The responses in the present study correspond with participants’ assessments of their own oral health. Those who assessed their oral health as good reported to a greater

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extent that oral health had a positive effect on their daily life. The results of the new subscales seem reasonable, as the great majority of respondents reported that oral health had an effect on the eating subscales but the effect on life in general was reported to a much lesser extent. The question used regarding self-rated oral health is an often used question and a five point Likert scale seems to give enough variety for the respondents to rate their oral health.

5.1.3 Questionnaires - reliability

A test is reliable to the extent that repeated measurements made under constant conditions will give the same results and is thus concerned with the degree of consistency or accuracy with which it measures an attribute (Moser, Kalton 1979, Polit, Hungler 1991). Measurement error plays a key role in reducing reliability; a reliable instrument minimizes the error component and maximizes the true component of a score. In Paper I, the age group 60-69 years reported an OHIP-14 score of 3.4 which is exactly the same as reported in Paper II (68-77 years of age), This strengthen the reliability of the OHIP-14.

Cronbach’s alpha was applied to assess the internal consistency reliability estimation in both Paper I and Paper III with acceptable result. In Paper III, the Cronbach`s alpha values of the subscales were between 0.75 and 0.90 indicating the high internal consistency reliability of the Dental Impact Profile inventory.

5.1.4 Clinical examination

The clinical examination was carried out by a dentist and a dental hygienist (KED) in a fully equipped dental clinic using a mirror and a probe. No radiographs were used, to enable comparison with previous studies and also because radiographs for epidemiological studies are seldom ethical, they expose people to radiation with no personal benefit. A calibration session was performed prior to the studies in which three patients were examined independently by the two examiners and the results were identical. The numbers of teeth present in the mouth and the numbers of teeth with dental caries were recorded. Dental caries was recorded using the DMFT index

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according to WHO criteria. A tooth was registered as decayed when caries extended into the dentine. The number of decayed teeth was used to indicate the caries status of the remaining teeth. This is a well known and commonly used index and the two examiners were experienced dental professionals, which strengthens the reliability of the clinical measurements. In Paper I, each respondent assessed the number of teeth present, a method which has been reported to be reliable (Bulin et al. 2002, Heløe 1972). The magnitude of the present correlations, the consistent findings and the conclusions harmonizing with the applied theories all indicate an acceptable reliability and validity of the findings.