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Anders Skrondal

Publications and source records attributed to Anders Skrondal.

10 recordsLinked to original sources
PubMed2006

A simulation study of three methods for detecting disease clusters.

BACKGROUND: Cluster detection is an important part of spatial epidemiology because it can help identifying environmental factors associated with disease and thus guide investigation of the aetiology of diseases. In this article we study three methods suitable for detecting local spatial clusters: (1) a spatial scan statistic (SaTScan), (2) generalized additive models (GAM) and (3) Bayesian disease mapping (BYM). We conducted a simulation study to compare the methods. Seven geographic clusters with different shapes were initially chosen as high-risk areas. Different scenarios for the magnitude of the relative risk of these areas as compared to the normal risk areas were considered. For each scenario the performance of the methods were assessed in terms of the sensitivity, specificity, and percentage correctly classified for each cluster. RESULTS: The performance depends on the relative risk, but all methods are in general suitable for identifying clusters with a relative risk larger than 1.5. However, it is difficult to detect clusters with lower relative risks. The GAM approach had the highest sensitivity, but relatively low specificity leading to an overestimation of the cluster area. Both the BYM and the SaTScan methods work well. Clusters with irregular shapes are more difficult to detect than more circular clusters. CONCLUSION: Based on our simulations we conclude that the methods differ in their ability to detect spatial clusters. Different aspects should be considered for appropriate choice of method such as size and shape of the assumed spatial clusters and the relative importance of sensitivity and specificity. In general, the BYM method seems preferable for local cluster detection with relatively high relative risks whereas the SaTScan method appears preferable for lower relative risks. The GAM method needs to be tuned (using cross-validation) to get satisfactory results.

Bayes Theorem↗
PubMed2006

Likelihood ratio tests in behavioral genetics: problems and solutions.

The likelihood ratio test of nested models for family data plays an important role in the assessment of genetic and environmental influences on the variation in traits. The test is routinely based on the assumption that the test statistic follows a chi-square distribution under the null, with the number of restricted parameters as degrees of freedom. However, tests of variance components constrained to be non-negative correspond to tests of parameters on the boundary of the parameter space. In this situation the standard test procedure provides too large p-values and the use of the Akaike Information Criterion (AIC) or the Bayesian Information Criterion (BIC) for model selection is problematic. Focusing on the classical ACE twin model for univariate traits, we adapt existing theory to show that the asymptotic distribution for the likelihood ratio statistic is a mixture of chi-square distributions, and we derive the mixing probabilities. We conclude that when testing the AE or the CE model against the ACE model, the p-values obtained from using the chi(2)(1 df) as the reference distribution should be halved. When the E model is tested against the ACE model, a mixture of chi(2)(0 df), chi(2)(1 df) and chi(2)(2 df) should be used as the reference distribution, and we provide a simple formula to compute the mixing probabilities. Similar results for tests of the AE, DE and E models against the ADE model are also derived. Failing to use the appropriate reference distribution can lead to invalid conclusions.

Analysis of Variance↗
PubMed2005

Recurrent risk of anal sphincter laceration among women with vaginal deliveries.

OBJECTIVE: The first aim of this study was to estimate the impact of anal sphincter laceration during the first delivery on the risk of recurrence in the second delivery. The second aim was to estimate the absolute risk of anal sphincter laceration in the second delivery according to the history of anal sphincter laceration and birth weight. METHODS: In this population-based cohort study, the study sample comprised all women included in the Norwegian Medical Birth Registry with 2 consecutive singleton vaginal deliveries during the period 1967-1998 (n = 486,463). The impact of prior anal sphincter laceration on recurrent anal sphincter laceration was estimated as crude and adjusted odds ratios (ORs). RESULTS: Anal sphincter laceration during first delivery increased the risk for a sphincter laceration in the next delivery, (adjusted OR 4.3, 95% confidence interval [CI] 3.8-4.8). Other risk factors were birth weight (adjusted OR 23.6, 95% CI 16.5-33.6, birth weight > 5,000 g versus birth weight < 3,000 grams), use of forceps (adjusted OR 5.1, 95% CI 4.3-6.0), use of vacuum (adjusted OR 1.4, 95% CI 1.1-1.7), and period of delivery (adjusted OR 4.3, 95% CI 3.7-5.0 for 1995-1998 versus 1967-1975). The absolute risks for anal sphincter laceration at second delivery for women with prior laceration were 1.3% (95% CI 0.4-3.2%) for birth weight less than 3,000 g and 23.3% (95% CI 11.8-38.6%) for birth weight more than 5,000 g. CONCLUSION: Only 10% of women with anal sphincter laceration at second delivery had a history of prior laceration. Prior anal sphincter laceration is associated with increased risk of laceration in second delivery, in particular in women who carry children with high birth weight. LEVEL OF EVIDENCE: II-2.

Adult↗
PubMed2005

Early respiratory infections, asthma, and allergy: 10-year follow-up of the Oslo Birth Cohort.

OBJECTIVE: It has been hypothesized that early infections protect against the development of atopic disease, but there have been few long-term follow-up studies. We estimated the associations between early respiratory infections and doctor-diagnosed asthma, allergic rhinitis, and skin-prick sensitization in children at 10 years of age in the Oslo Birth Cohort, established in 1992-1993. We also considered birth order and attendance at a child care center as proxy measures of increased exposure to infections early in life. METHODS: A total of 2540 children were followed from birth to the age of 10 years. Experiences of respiratory infections were recorded in follow-up surveys at 6 and 12 months. At age 10, questions were asked about current symptoms of asthma and allergic rhinitis and about having ever received a doctor diagnosis for these diseases. A subsample (n = 1740) of the cohort was tested for skin-prick test reactivity. RESULTS: Current asthma was related to lower respiratory tract infection (adjusted odds ratio [OR]: 2.1; 95% confidence interval: 1.3-3.0) and croup (adjusted OR: 2.3; 95% confidence interval: 1.3-4.2) in the first year. ORs for allergic rhinitis and skin-prick sensitization were smaller but mainly positive. Birth order and child care attendance at age 1 year were not significantly associated with any of the studied outcomes. CONCLUSIONS: Early respiratory infections did not protect against the development of asthma, allergic rhinitis, or sensitization to common allergens during the first 10 years of life but increased the risk for asthma symptoms at age 10 in this population.

Allergens↗
PubMed2004

Recurrence risk of preeclampsia in twin and singleton pregnancies.

The etiology of preeclampsia is unknown. The relatively high risk of recurrence of preeclampsia in subsequent pregnancies to the same mother suggests a genetic basis for the disease, but the mode of inheritance is uncertain. We compare the risk of preeclampsia in second pregnancies for mothers whose first preeclamptic pregnancy was either a singleton or a twin pregnancy. The crude and adjusted recurrence risks of preeclampsia in twin and singleton pregnancies were estimated in a population-based register including the first and second pregnancies of 550218 women registered in the Medical Birth Registry of Norway, 1967-1998. The recurrence risk of preeclampsia in second pregnancy for women with a singleton pregnancy with preeclampsia the first time was 14.1% (95% CI: 13.6-14.6). For women with a first time twin pregnancy the recurrence risk was lower, 6.8% (CI: 4.3-10.1), P < 0.001. Thus, the crude excess risk for recurrent preeclampsia was 7.3% (95% CI: 4.5-10.0) in women with a first time singleton as compared to women with a first time twin pregnancy. The recurrence risk of preeclampsia is lower when the first pregnancy was a twin as compared to a singleton pregnancy. This observation is consistent with a polygenic liability model.

Adult↗
PubMed2004

The importance of social characteristics of communities for the medically based disability pension.

BACKGROUND: The aim of this study was to look for any possible contextual effect of deprivation at municipality level on the risk of being granted the medically based disability pension, controlled for compositional effects due to spatial concentration of people with a high risk of disability. METHODS: The material consists of the residentiary part of a total Norwegian county population aged 20-54 years without disability pension at baseline, n=40,083. This study was performed as a 10-year follow-up study. The relative risk of being granted a disability pension was estimated by logistic regression analyses as odds ratios (OR) between people living in different municipalities according to a municipality deprivation index at three levels, adjusted for individual factors. RESULTS: The OR of disability pension was 1.36 (1.22, 1.51) for people residing in intermediate deprived municipalities and 1.48 (1.31, 1.67) for people residing in the most deprived municipalities compared to the most affluent municipalities, adjusted for gender and age. After adjustment for individual risk factors the OR was 1.26 (1.12, 1.41) and 1.18 (1.04, 1.35) respectively. Analyses stratified by gender showed that the increased risk of receiving a disability pension for men in the most deprived municipalities was explained by individual factors alone. CONCLUSION: Relative municipality deprivation seems to account for an increase in the incidence of disability pension. This effect contributes to marginalization of people living in less affluent areas out of employment and thus to widening socioeconomic inequalities in the population.

Adult↗
PubMed2003

Interaction as departure from additivity in case-control studies: a cautionary note.

It has been argued that assessment of interaction should be based on departures from additive rates or risks. The corresponding fundamental interaction parameter cannot generally be estimated from case-control studies. Thus, surrogate measures of interaction based on relative risks from logistic models have been proposed, such as the relative excess risk due to interaction (RERI), the attributable proportion due to interaction (AP), and the synergy index (S). In practice, it is usually necessary to include covariates such as age and gender to control for confounding. The author uncovers two problems associated with surrogate interaction measures in this case: First, RERI and AP vary across strata defined by the covariates, whereas the fundamental interaction parameter is unvarying. S does not vary across strata, which suggests that it is the measure of choice. Second, a misspecification problem implies that measures based on logistic regression only approximate the true measures. This problem can be rectified by using a linear odds model, which also enables investigators to test whether the fundamental interaction parameter is zero. A simulation study reveals that coverage is much improved by using the linear odds model, but bias may be a concern regardless of whether logistic regression or the linear odds model is used.

Case-Control Studies↗
PubMed2003

A comparison of anxiety and depressive symptomatology in postpartum and non-postpartum mothers.

BACKGROUND: The aim of the study was to compare the distribution of symptoms of anxiety and depression among postpartum as compared with non-postpartum mothers, and to estimate the impact of the postpartum period on the risk of anxiety and depression when adjusted for other risk factors. METHODS: A questionnaire survey approaching all women 18-40 years of age in two municipalities in Norway during the period 1998-1999 was conducted. A total of 2,730 women were included, of whom 416 were in the postpartum period. Only women with one or more deliveries (n = 1,794) were included in the multivariate analyses. RESULTS: The crude prevalence of anxiety and depression was lower in postpartum as compared to non-postpartum mothers. However, when controlling for other risk factors, the odds ratio for anxiety was 1.2 (95% CI: 0.6-2.3) and for depression 1.8 (95% CI: 1.1-2.9) during the postpartum period. CONCLUSION: The overall risk of anxiety appeared to be the same in both groups, whereas the risk of depression was increased in the postpartum group.

Adolescent↗
PubMed2003

Traffic related air pollution and acute hospital admission for respiratory diseases in Drammen, Norway 1995-2000.

The aim of this study was to estimate the associations between seven ambient air pollutants [particulate matter (PM10), nitrous dioxide (NO2), sulfur dioxide (SO2), ozone (O3), benzene, formaldehyde and toluene] and acute hospital admissions for respiratory diseases in Drammen, Norway 1995-2000. Time-series analysis of counts was performed by means of generalized additive models with log link and Poisson distribution. The results showed that benzene was the pollutant having the strongest association with respiratory diseases for the total study period, the relative risk of an interquartile increase of benzene was 1.095 with 95% confidence interval: 1.031-1.163. The corresponding results were 1.049 (0.990-1.112) for formaldehyde, 1.044 (1.000-1.090) for toluene, 1.064 (1.019-1.111) for NO2, 1.043 (1.011-1.075) for SO2, 0.990 (0.936-1.049) for O3 and 1.022 (0.990-1.055) for PM10. Dividing the total study period into two 3-year periods, there was a substantial reduction in the exposure levels of the volatile organic compounds (benzene, formaldehyde and toluene) from the first to the second period. Separate analyses for the second time period showed weaker association between these pollutants and the health outcome. This study provides further evidence for short-term respiratory health effects of traffic related air pollution.

Acute Disease↗