Self-rated health and its bidirectional relationship with burnout, sleep quality and somatic symptoms in a general adult population.

The aim of this study was to investigate how self-rated health (SRH) reflects ongoing ill-health and how SRH is associated with previous ill-health and/or predicts future ill-health such as burnout, disturbed sleep, and somatic symptoms. The study used two waves from the population-based Västerbotten Environmental and Health Study in which 2 336 adult persons participated by answering a questionnaire at two time points three years apart. Hierarchical and logistic regression analyses were conducted, thus treating all variables both continuously (degree) and categorically (case). The analyses were performed both cross-sectionally and longitudinally. The results showed bidirectionality between suboptimal SRH and burnout, disturbed sleep and somatic severity caseness. Moreover, degree of poor SRH was more likely to occur simultaneously to high degrees of burnout and somatic severity than to degree of poor sleep quality. Also, caseness of burnout, disturbed sleep and somatic severity increased the risk of simultaneous suboptimal SRH. Finally, the results showed that degree of burnout three years earlier, predicted degree of poor SRH, and that degree of poor SRH predicted degree of sleep three years later. In conclusion, in a population-based, normal adult sample there is a bidirectional relationship between suboptimal SRH and caseness of burnout, disturbed sleep quality and somatic symptoms, but not between degree of these symptoms. The results can have implications for health care meeting patients complaining about poor general health.

Albumin and fibrinogen synthesis rates in advanced chronic liver disease.

Synthesis of plasma proteins is an important function of the liver that has sparsely been investigated by modern techniques in patients with advanced chronic liver disease (CLD). Twenty-eight well-characterized patients with CLD under evaluation for liver transplantation were included. Albumin and fibrinogen synthesis rates were measured by the flooding dose technique using stable isotope-labeled phenylalanine. Transcapillary escape rate of albumin and plasma volume were assessed by radioiodinated human serum albumin. The absolute albumin synthesis rates were low (65 mg/kg/day, range: 32-203) and were associated with impaired liver function, as reflected by the risk-scores Child-Pugh (P = 0.025) and model for end-stage liver disease (rs = -0.62, P = 0.0005). The fibrinogen synthesis rate (12.8 mg/kg/day, range: 2.4-52.9) was also negatively associated with liver function. The synthesis rates of albumin and fibrinogen were positively correlated. Plasma volume was high (51 ± 9 mL/kg body wt), which contributed to an almost normal intravascular albumin mass despite low plasma concentration. Autoimmune inflammatory etiologies to CLD were associated with higher fibrinogen synthesis. De novo synthesis rates of albumin and fibrinogen in advanced chronic liver failure were negatively correlated to prognostic scores of liver disease. Albumin synthesis rate was low and associated with both liver failure and autoimmune inflammation, whereas fibrinogen synthesis was often normal and positively associated with chronic inflammation. This is different from acute inflammatory states in which both albumin and fibrinogen synthesis rates are high.NEW & NOTEWORTHY Albumin and fibrinogen synthesis were positively correlated, but the high variation indicates that these are probably influenced by different mechanisms. There might be a limited metabolic reserve for the liver to increase both albumin and fibrinogen synthesis in response to longstanding inflammation in CLD and fibrinogen seems to be prioritized.

Memory profiles predict dementia over 23-28 years in normal but not successful aging.

OBJECTIVES: Prospective studies suggest that memory deficits are detectable decades before clinical symptoms of dementia emerge. However, individual differences in long-term memory trajectories prior to diagnosis need to be further elucidated. The aim of the current study was to investigate long-term dementia and mortality risk for individuals with different memory trajectory profiles in a well-characterized population-based sample. METHODS: 1062 adults (aged 45-80 years) who were non-demented at baseline were followed over 23-28 years. Dementia and mortality risk were studied for three previously classified episodic memory trajectory groups: maintained high performance (Maintainers; 26%), average decline (Averages; 64%), and accelerated decline (Decliners; 12%), using multistate modeling to characterize individuals' transitions from an initial non-demented state, possibly to a state of dementia and/or death. RESULTS: The memory groups showed considerable intergroup variability in memory profiles, starting 10-15 years prior to dementia diagnosis, and prior to death. A strong relationship between memory trajectory group and dementia risk was found. Specifically, Decliners had more than a fourfold risk of developing dementia compared to Averages. In contrast, Maintainers had a 2.6 times decreased dementia risk compared to Averages, and in addition showed no detectable memory decline prior to dementia diagnosis. A similar pattern of association was found for the memory groups and mortality risk, although only among non-demented. CONCLUSION: There was a strong relationship between accelerated memory decline and dementia, further supporting the prognostic value of memory decline. The intergroup differences, however, suggest that mechanisms involved in successful memory aging may delay symptom onset.

Examining the psychometric properties of the Swedish version of Clinical Outcomes in Routine Evaluation-Outcome Measure (CORE-OM) in a clinical sample using classical test theory and item response theory.

The aim of this study was to examine the psychometric properties of the Swedish version of the Clinical Outcomes in Routine Evaluation-Outcome Measure (CORE-OM) using classical test theory and item response theory (IRT). The CORE-OM is a commonly used 34-item self-report instrument measuring psychological problems/distress covering four domains: subjective well-being, problems/symptoms, functioning and risk. Despite its broad application, only a few studies have used IRT to examine the psychometric properties, and the properties of the Swedish version have only been examined in one initial study. The present study included 1,011 clients with mild to moderate symptoms of distress, applying for psychotherapy at an outpatient training clinic in Sweden. Clients' responses were subjected to classical item analyses as well as IRT (Rasch) analysis using the partial credit model. The classical analyses demonstrated high levels of internal consistency and acceptable levels of item discrimination for the majority of the items, although lower for some items, particularly in the Risk domain. IRT analyses showed that there was a rather good match between item and respondent locations and the measurement precision was high. Disordered step and average measures for some of the items in the Risk domain indicate that these items were problematic from a psychometric point of view and only applicable for a minority of the participants. Differential item functioning for gender in some of the items suggests that they might need to be revised to minimise potential gender bias.

A nationwide Swedish study of age at retirement and dementia risk.

OBJECTIVES: The aim of this nationwide study was to examine the association between age at retirement and dementia risk, with a follow-up period of up to 24 years. METHODS/DESIGN: This cohort study comprised Swedish citizens born in 1930 who were alive in the year 1990 (n = 63 505). The cohort was followed for incidents of dementia through data provided by the Swedish National Patient Register and the Cause of Death Register. Age at retirement and socioeconomic variables were retrieved from Statistics Sweden. RESULTS: During the follow-up, 5181 individuals received a dementia diagnosis. Competing risk regression models, adjusted for sex, education, marital status, occupation, and previous history of cardiovascular diseases, showed that later-than-average retirement age was associated with decreased dementia risk. CONCLUSIONS: The present results support the idea that individuals who retired at an older age have a decrease risk of dementia. However, as this was an observation study, unmeasured factors, such as premorbid cognitive level and genetic predisposition, may have influenced our findings and remains to be elucidated in future studies.

Road traffic noise, air pollution, and risk of dementia - results from the Betula project.

BACKGROUND: There is growing evidence for a negative impact of traffic-related air pollution on risk of dementia. However, the contribution of noise exposure to this association has been rarely examined. OBJECTIVE: We aimed to investigate the individual and combined effect of noise and air pollution on risk of dementia. METHODS: Data on dementia incidence over a 15 year period was obtained from the Betula project, a longitudinal study on health and ageing. Estimates of annual mean levels of nitrogen oxides (NOx) at the participants' residential address were obtained using a land-use regression model. Modelled data provided road traffic noise levels (Leq. 24 h) at the participants' residential address at baseline. Cox proportional hazard regression was used to calculate hazard ratios (HR). RESULTS: Of 1721 participants at baseline, 302 developed dementia during the follow up period. Exposure to noise levels (Leq. 24 h) > 55 dB had no significant effect on dementia risk (HR 0.95; CI: 0.57, 1.57). Residing in the two highest quartiles of NOx exposure was associated with an increased risk of dementia. The risk associated with NOx was not modified by adjusting for noise. Moreover, we found no significant interaction effects between NOx and road traffic noise on dementia risk. CONCLUSION: We found no evidence that exposure to road traffic noise, either independently or in combination with traffic air pollution, was associated with risk of dementia in our study area. Our results suggest that pollution should be considered the main component in the association between traffic related exposures and dementia.

Social relationships and risk of dementia: a population-based study.

BACKGROUND: The objective was to examine whether aspects of social relationships in old age are associated with all-cause dementia and Alzheimer's disease (AD). METHODS: We studied 1,715 older adults (≥ 65 years) who were dementia-free at baseline over a period of up to 16 years. Data on living status, contact/visit frequency, satisfaction with contact frequency, and having/not having a close friend were analyzed using Cox proportional hazards regressions with all-cause dementia or AD as the dependent variable. To control for reverse causality and to identify potential long-term effects, we additionally performed analyses with delayed entry. RESULTS: We identified 373 incident cases of dementia (207 with AD) during follow-up. The variable visiting/visits from friends was associated with reduced risk of all-cause dementia. Further, a higher value on the relationships index (sum of all variables) was associated with reduced risk of all-cause dementia and AD. However, in analyses with delayed entry, restricted to participants with a survival time of three years or more, none of the social relationship variables was associated with all-cause dementia or AD. CONCLUSIONS: The results indicate that certain aspects of social relationships are associated with incident dementia or AD, but also that these associations may reflect reverse causality. Future studies aimed at identifying other factors of a person's social life that may have the potential to postpone dementia should consider the effects of reverse causality.

Subjective memory impairment in older adults predicts future dementia independent of baseline memory performance: Evidence from the Betula prospective cohort study.

INTRODUCTION: The objective was to examine whether subjective memory impairment (SMI) predicts all-cause dementia or Alzheimer's disease (AD) in a population-based study with long-term follow-up (median = 10 years). METHODS: A total of 2043 initially dementia-free participants (≥ 60 years) made three memory ratings ("compared with others", "compared with five years ago", and "complaints from family/friends") at baseline. During follow-up, 372 participants developed dementia (208 with AD). RESULTS: Cox regression revealed that subjective memory impairment ratings predicted all-cause dementia in models adjusting for age and sex (hazard ratio or HR from 2.04 to 3.94), with even higher values for AD (HR from 2.29 to 5.74). The result persisted in models including other covariates, including baseline episodic memory performance, and in analyses restricted to participants with long time to dementia diagnosis (≥ 5 years). DISCUSSION: The findings underscore the usefulness of subjective memory assessment in combination with other factors in identifying individuals at risk for developing dementia.

Stressful life events are not associated with the development of dementia.

BACKGROUND: The impact of stressful life events as a risk factor of dementia diseases is inconclusive. We sought to determine whether stressful negative life events are associated with incidental dementia in a population-based study with long-term follow-up. We also tested the hypothesis that the occurrence of positive life events could mitigate or overcome the possible adverse effects of negative life events on dementia conversion. METHODS: The study involved 2,462 dementia-free participants aged 55 years and older. Information on life events was ascertained at baseline from a comprehensive Life Event Inventory, which included 56 questions about specific life events. For each life event, the emotional impact (both positive and negative) and emotional adjustment were asked for. RESULTS: During follow-up, 423 participants developed dementia; of these, 240 developed Alzheimer's disease (AD). Cox regression analysis showed no association between the total number of negative life events and the incidence of dementia when adjusted solely for age and gender (hazard ratio = 0.97, 95% CI = 0.92-1.02), or with multiple adjustments for a range of covariates (hazard ratio = 0.96, 95% CI = 0.91-1.01). Similarly, neither emotional impact nor emotional adjustment to these life events was associated with incident dementia. A separate analysis of AD did not alter the results. CONCLUSIONS: The result of this population-based study finds no association between negative or positive life events and dementia. Accordingly, our results reject the hypothesis that stressful life events trigger the onset of dementia diseases.

The relationship between marital and parental status and the risk of dementia.

BACKGROUND: This study examines the association between marital and parental status and their individual and combined effect on risk of dementia diseases in a population-based longitudinal study while controlling for a range of potential confounders, including social networks and exposure to stressful negative life events. METHODS: A total of 1,609 participants without dementia, aged 65 years and over, were followed for an average period of 8.6 years (SD = 4.8). During follow-up, 354 participants were diagnosed with dementia. Cox regression was used to investigate the effect of marital and parental status on risk of dementia. RESULTS: In univariate Cox regression models (adjusted for age as time scale), widowed (hazard ratio (HR) 1.42, 95% confidence interval (CI) = 1.13-1.78), and not having children (HR 1.54, 95% CI = 1.15-2.06) were significantly associated with incident dementia. In multivariate analyses that included simultaneously marital and parental status and covariates that were found to be significant in univariate models (p < 0.10), the HR was 1.30 (95% CI = 1.01-1.66) for widowed, and 1.51 (95% CI = 1.08-2.10) for those not having children. Finally, a group of four combined factors was constructed: married parents (reference), married without children, widowed parents, and widowed without children. The combined effect revealed a 1.3 times higher risk (95% CI = 1.03-1.76) of dementia in widow parents, and a 2.2 times higher risk (95% CI = 1.36-3.60) in widowed persons without children, in relation to married parents. No significant difference was observed for those being married and without children. CONCLUSIONS: Our findings suggest that marital- and parental status are important risk factors for developing dementia, with especially increased risk in those being both widowed and without children.

Effects of perceived long-term stress on subjective and objective aspects of memory and cognitive functioning in a middle-aged population-based sample.

The longitudinal effects of perceived stress on measures of memory and two other cognitive functions (word fluency, visuospatial ability) in a middle-aged sample (40-60 years, M age = 47.1 years, SD = 6.1 years; n = 192) were examined. A group describing themselves as stressed in general at baseline, and at follow-up measurement 5 and 10 years later (n = 96) was compared with a matched (age, sex) low-stress group (n = 96). The results revealed more depressive symptoms over time in the high-stress group. With regard to memory, a dissociation between subjective and objective measures was observed. Specifically, participants in the high-stress group rated their memory as worse over time as compared with controls, and reported a higher frequency of occurrence of everyday memory failures, effects partly independent of depressive symptoms. However, the groups did not differ in terms of objective episodic memory performance, word fluency or block design performance, with stable levels of performance over time regardless of perceived stress. The lack of effects of stress on cognitive performance is discussed in the light of factors such as stress level, age of the participants, and other individual difference factors.

Construct validation and normative data for different versions of the Shirom-Melamed burnout questionnaire/measure in a Swedish population sample.

The overall aim of the present study was to examine the construct validity of different versions of the Shirom-Melamed Burnout Questionnaire/Measure (SMBQ/M) suggested in previous work, as well as to provide normative data for a large population-based sample in Sweden with a broad range in age. Cross-sectional data from the Västerbotten Environmental Health Study in Sweden were used. The 3406 participants (56% women) in this study, aged 18-79 years, constituted a random sample stratified for age and sex. Participants responded to a questionnaire including the 4-factor 22-item version of the SMBM as well as background questions and a number of validated questionnaire instruments assessing for example, sleep disturbance, depression, anxiety, perceived stress, and somatic symptoms. The dimensionality of different versions of the SMBM were examined with confirmatory factor analysis. A modified 4-factor 19-item model of the SMBM provided good model-fit, and two 2-factor models (11-item and 6-item) provided excellent model fit. The relationships to relevant psychological constructs provided support for convergent validity for the suggested versions of the SMBM. Finally, normative data were obtained for a broad age group for the different versions. In conclusion, we suggest that for assessing the core of the burnout construct in terms of emotional and physical exhaustion and cognitive weariness, the SMBM-11 or SMBM-6 for repeated measures, are to be used. For a broader assessment of burnout incorporating both symptoms and information about the process of exhaustion via the subscales of listlessness and tension, we recommend the use of the modified 4-factor SMBM-19.

PM2.5 and Dementia in a Low Exposure Setting: The Influence of Odor Identification Ability and APOE.

BACKGROUND: Growing evidence show that long term exposure to air pollution increases the risk of dementia. OBJECTIVE: The aim of this study was to investigate associations between PM2.5 exposure and dementia in a low exposure area, and to investigate the role of olfaction and the APOE ɛ4 allele in these associations. METHODS: Data were drawn from the Betula project, a longitudinal study on aging, memory, and dementia in Sweden. Odor identification ability was assessed using the Scandinavian Odor Identification Test (SOIT). Annual mean PM2.5 concentrations were obtained from a dispersion-model and matched at the participants' residential address. Proportional hazard regression was used to calculate hazard ratios. RESULTS: Of 1,846 participants, 348 developed dementia during the 21-year follow-up period. The average annual mean PM2.5 exposure at baseline was 6.77µg/m3, which is 1.77µg/m3 above the WHO definition of clean air. In a fully adjusted model (adjusted for age, sex, APOE, SOIT, cardiovascular diseases and risk factors, and education) each 1µg/m3 difference in annual mean PM2.5-concentration was associated with a hazard ratio of 1.23 for dementia (95% CI: 1.01-1.50). Analyses stratified by APOE status (ɛ4 carriers versus non-carriers), and odor identification ability (high versus low), showed associations only for ɛ4 carriers, and for low performance on odor identification ability. CONCLUSION: PM2.5 was associated with an increased risk of dementia in this low pollution setting. The associations between PM2.5 and dementia seemed stronger in APOE carriers and those with below average odor identification ability.

Dengue and Zika RNA-RNA interactomes reveal pro- and anti-viral RNA in human cells.

BACKGROUND: Identifying host factors is key to understanding RNA virus pathogenicity. Besides proteins, RNAs can interact with virus genomes to impact replication. RESULTS: Here, we use proximity ligation sequencing to identify virus-host RNA interactions for four strains of Zika virus (ZIKV) and one strain of dengue virus (DENV-1) in human cells. We find hundreds of coding and non-coding RNAs that bind to DENV and ZIKV viruses. Host RNAs tend to bind to single-stranded regions along the virus genomes according to hybridization energetics. Compared to SARS-CoV-2 interactors, ZIKV-interacting host RNAs tend to be downregulated upon virus infection. Knockdown of several short non-coding RNAs, including miR19a-3p, and 7SK RNA results in a decrease in viral replication, suggesting that they act as virus-permissive factors. In addition, the 3'UTR of DYNLT1 mRNA acts as a virus-restrictive factor by binding to the conserved dumbbell region on DENV and ZIKV 3'UTR to decrease virus replication. We also identify a conserved set of host RNAs that interacts with DENV, ZIKV, and SARS-CoV-2, suggesting that these RNAs are broadly important for RNA virus infection. CONCLUSIONS: This study demonstrates that host RNAs can impact virus replication in permissive and restrictive ways, expanding our understanding of host factors and RNA-based gene regulation during viral pathogenesis.

The effects of marital status on episodic and semantic memory in healthy middle-aged and old individuals.

The present study examined the influences of marital status on different episodic and semantic memory tasks. A total of 1882 adult men and women participated in a longitudinal project (Betula) on memory, health and aging. The participants were grouped into two age cohorts, 35-60 and 65-85, and studied over a period of 5 years. Episodic memory tasks concerned recognition and recall, whereas semantic memory tasks concerned knowledge and fluency. The results showed, after controlling for education, some diseases, chronological age and leisure activity as covariates, that there were significant differences between married and single individuals in episodic memory, but not in semantic memory. Married people showed significantly better memory performances than singles in both subsystems of episodic memory, that is, recall and recognition. Also, the rate of decline in episodic memory was significantly larger for singles and widowed than other groups over the 5-year time period in both age groups. The findings demonstrate that the positive relation found between marriage and health can be extended to the relation between marriage and cognitive performance. This effect might be explained by the role played by cognitive stimulation in memory and cognition.

Examining Skills and Abilities During the Pandemic - Psychology Students' and Examiners' Perceptions of a Digital OSCE.

Finding valid and reliable ways to assess complex clinical skills within psychology is a challenge. Recently, there have been some examples of applying Objective Structured Clinical Examinations (OSCEs) in psychology for making such assessments. The aim of this study was to examine students' and examiners' perceptions of a digital OSCE in psychology regarding quality and students' feelings about the OSCE. Participants were 51 students enrolled in the Programme for Master of Science in Clinical Psychology during two semesters and nine examiners assessing each OSCE occasion, at Umeå University, Sweden. Web-based questionnaires were used for data collection. Psychometric analyses indicated that the subscales in the student questionnaire had adequate or close to adequate levels of item and scale reliability. Both students and examiners felt that the digital OSCE was realistic, valid and well-aligned with professional practice. Although students perceived the digital OSCE as stressful, the results showed that they were focused and concentrated and found the OSCE to be a positive learning experience, implying that the stress did not affect performance to any significant extent. Based on the examiners' experiences, it can be concluded that there are both advantages and disadvantages which need to be considered when planning future digital OSCEs.

Loneliness Increases the Risk of All-Cause Dementia and Alzheimer's Disease.

OBJECTIVES: To examine the effect of perceived loneliness on the development of dementia (all-cause), Alzheimer´s disease (AD), and vascular dementia (VaD). METHOD: The study comprised 1,905 nondemented participants at baseline, drawn from the longitudinal Betula study in Sweden, with a follow-up time of up to 20 years (mean 11.1 years). Loneliness was measured with a single question: "Do you often feel lonely?". RESULTS: During the follow-up, 428 developed dementia; 221 had AD, 157 had VaD, and 50 had dementia of other subtypes. The entire dementia group is denoted "all-cause dementia." Cox regression models, adjusted for age, gender, and a baseline report of perceived loneliness, showed increased risk of all-cause dementia (hazard ratio [HR] = 1.46, 95% confidence interval [CI] 1.14-1.89), and AD (HR = 1.69, 95% CI 1.20-2.37), but not VaD (HR = 1.34, 95% CI 0.87-2.08). After adjusting for a range of potential confounders, and excluding participants with dementia onset within the first 5 years of baseline (to consider the possibility of reverse causality), the increased risk for the development of all-cause dementia and AD still remained significant (HR = 1.51, 95% CI 1.01-2.25 for all-cause dementia; HR = 2.50, 95% CI 1.44-4.36 for AD). DISCUSSION: The results suggest that perceived loneliness is an important risk factor for all-cause dementia and especially for AD, but not for VaD. These results underscore the importance of paying attention to subjective reports of loneliness among the elderly adults and identifying potential intervention strategies that can reduce loneliness.

Mental Demands at Work and Risk of Dementia.

High mental demands at work was examined as a possible protective factor to reduce the risk of dementia in 1,277 initially dementia-free participants, aged 60 years and older. The cohort was followed for a mean of 13.6 years. During follow-up, 376 participants developed all-cause dementia (Alzheimer's disease = 199; vascular dementia = 145). The association between mental demands at work and dementia was analyzed with Cox hazard models, adjusted for a range of covariates. The results revealed no significant association between mental demands at work and incidence of dementia. Based on the measures used in this study, it was concluded that high mental demands at work may not reduce the risk of dementia later in life.

Biological and environmental predictors of heterogeneity in neurocognitive ageing: Evidence from Betula and other longitudinal studies.

Individual differences in cognitive performance increase with advancing age, reflecting marked cognitive changes in some individuals along with little or no change in others. Genetic and lifestyle factors are assumed to influence cognitive performance in ageing by affecting the magnitude and extent of age-related brain changes (i.e., brain maintenance or atrophy), as well as the ability to recruit compensatory processes. The purpose of this review is to present findings from the Betula study and other longitudinal studies, with a focus on clarifying the role of key biological and environmental factors assumed to underlie individual differences in brain and cognitive ageing. We discuss the vital importance of sampling, analytic methods, consideration of non-ignorable dropout, and related issues for valid conclusions on factors that influence healthy neurocognitive ageing.

Traffic-Related Air Pollution as a Risk Factor for Dementia: No Clear Modifying Effects of APOEɛ4 in the Betula Cohort.

It is widely known that the apolipoprotein E (APOE) ɛ4 allele imposes a higher risk for Alzheimer's disease (AD). Recent evidence suggests that exposure to air pollution is also a risk factor for AD, and results from a few studies indicate that the effect of air pollution on cognitive function and dementia is stronger in APOEɛ4 carriers than in non-carriers. Air pollution and interaction with APOEɛ4 on AD risk thus merits further attention. We studied dementia incidence over a 15-year period from the longitudinal Betula study in Northern Sweden. As a marker for long-term exposure to traffic-related air pollution, we used modelled annual mean nitrogen oxide levels at the residential address of the participants at start of follow-up. Nitrogen oxide correlate well with fine particulate air pollution levels in the study area. We had full data on air pollution, incidence of AD and vascular dementia (VaD), APOEɛ4 carrier status, and relevant confounding factors for 1,567 participants. As expected, air pollution was rather clearly associated with dementia incidence. However, there was no evidence for a modifying effect by APOEɛ4 on the association (p-value for interaction > 0.30 for both total dementia (AD+VaD) and AD). The results from this study do not imply that adverse effects of air pollution on dementia incidence is limited to, or stronger in, APOEɛ4 carriers than in the total population.