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Introduction to Methods in Psychiatric Epidemiology. Matias Irarrazaval MD MPH University of Chile RedeAmericas February 18, 2015. Intended Audience. This lecture is designed to give students a general introduction to epidemiologic theory and research methods.
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Introduction to Methods in Psychiatric Epidemiology Matias Irarrazaval MD MPH University of Chile RedeAmericas February 18, 2015
Intended Audience This lecture is designed to give students a general introduction to epidemiologic theory and research methods. No previous knowledge of epidemiology is required.
Learning Objectives By the end of this lecture, users will be able to: • Appreciate the importance of epidemiological research methods in psychiatric epidemiology. • Understand relevant summary measures of disease frequency and disease burden.
Outline • Psychiatric epidemiology: definition & importance • Basic terminology • Mental disorder, case, population, observation time • Measurement or information error • Random and systematic error • Measures of disease frequency • Prevalence and incidence • Measures of disease burden • Mortality, morbidity, financial costs, quality-adjusted life years (QALYs), disability-adjusted life years (DALYs)
What is Psychiatric Epidemiology? • The study of the distribution and determinants of mental disorders or health-related events in specified populations, and the application of this study to the control of mental health problems. • Uses the principles of epidemiology in understanding the onset, course (duration and recurrence), and consequences of mental disorders. • A key purpose is to discover causes of disorders and ways of preventing disorders.
Why it is Important? • To assess burden of mental disorders in populations. • To make significant comparisons of mental disorders frequency between • Disorders (e.g., causes of death) • Population subgroups (e.g., children/adults) • Places (e.g., urban/rural) • Time periods (e.g., before/after intervention)
What is a Mental Disorder? ICD-10 Definition (WHO) • Clinically recognizable set of symptoms or behaviour associated in most cases with distress and interference with personal functions. • Social deviance or conflict alone, without personal dysfunction, should not be included in mental disorder as defined here.
What is a Case? • A subject that has a condition of interest. • Case definition may be different from clinical diagnosis. • Problems defining “caseness” in psychiatry: • Often there are no objectively assessable measures to establish the presence or absence of a given syndrome. • Debates about the validity of diagnostic categories.
What is a Population • Defined population: A group of individuals that can be defined by certain shared characteristics (e.g., adults with schizophrenia). • Base or denominator for a set of cases. • Closed population: Membership in the population that does not change over the period of observation (e.g., participants in a study with complete follow-up, famine survivors). • Open populations: Membership is based on a condition and it is transitory. The population can gain or lose members over time due (e.g., resi- • dents of a city, hospital patients).
What is an Observation Time? • Period of observation for each person
Measurement or Information Error • Random error: The divergence, by chance alone, of a measurement from the true value. • For example, if those who agree to take part in a study are more likely to be depressed than those who do not agree to take part. • Quantifiable with the confidence interval. • Can be decreased using a larger sample size and reducing the variability in measurement.
Measurement or Information Error • Systematic error: When observations in a study differ from the true value in a non-random way. • Examples: Bias and confounding: • For example, if those who agree to take part in a study are more likely to be depressed than those who do not agree to take part (selection bias).
Measures of Disease Frequency • Measures of disease frequency are used to describe how common an illness (or other health event) is with reference to the size of the population (the population at risk) and a measure of time. • There are two main measures of disease frequency:
Prevalence • Prevalence is a measure of the occurrence of any type of health condition, exposure, or other factor related to health. • There are two main measures of prevalence:
Point Prevalence • Point prevalence: Frequency of a disease or condition in a defined population at a given point of time • Example: • Of 10,000 adults in town A, on 1st June 2007, 2,000 have depression. The prevalence of depression among adults in town A on June 1st, 2007 is = 2,000/10,000 = 0.2 or 20% Point Number of people with disease at a given point of time Prevalence Number of people in the population at the same point of time =
Period Prevalence • Period prevalence:Proportion of individuals with the condition at any time during a specified time period or interval. • Example: • In 2012, the 12-month prevalence for major depressive episode among U.S. adults was 6.9%. Period Number of people with the condition in a given period of time Prevalence Number of people in the population during same period of time =
Lifetime Prevalence (%) of Mental Disorders in Different Countries in 2002(WHO World Mental Health Survey, 2007)
Incidence • Incidence: Number of new cases of a disease (or other health outcome of interest) that develops in a population at risk during a specified time period. • There are two main measures of incidence:
Incidence Rate • Incidence rate: Number of new cases within a specified time period divided by the person-time at risk during the time period. • Example: • 11 new cases of autism are diagnosed in a community during 2007. In June, the population of the community is 100,000 people. • Incidence rate = 11 cases/100,000 persons x 1 year • = 11/100,000 person-years, or, 1 per 10,000 person-years Incidence Number of people who develop disease in a specific time period Rate Person-time units at risk during the follow-up period =
Incidence during 2007: 1 new case/10 persons x 1 year = 10 per 100 person-years
Incidence Risk (Cumulative Incidence) • Incidence risk: Proportion of individuals in a population (initially free of disease) who develop the disease within a specified time interval. • Example: • 100 people in a closed population (i.e. adds no new members over time) are observed for a 2-year period. 2 develop autism during this period. The 2-year cumulative incidence of autism in this population is= (2/100)*100= 2 per 100 persons or 2%. Incidence Number of people who develop disease in a specific time period Risk Number of disease-free persons at beginning of that time period =
Measures of Disease Burden • Disease burden: Impact of a health problem within a given geographical area or population. • Measures: • Mortality (mortality rates, survival rates). • Morbidity: any departure, subjective or objective, from a state of physiological or psychological well-being. • Financial Costs. • Composite indicators: QALY, DALY.
Mortality and Morbidity • Crude mortality rate: Total number of deaths from all causes in a given population (usually per 100.000 population per year). • E.g. In USA, during 2007, the crude mortality rate was of 803.6/100,000 population year. • Cause-specific mortality (or death) rate: Number of deaths from a specific cause per 100,000 population per year. • E.g.. In South Korea, during 2012, the suicide mortality rate was of 28.1/100,000 population year.
Mortality and Morbidity • Morbidity rate: Number of existing or new cases of a particular disease or condition per 100 population. • E.g. in 2000, the mental morbidity rate among Nigerian adults aged 60 years and over was 45.3%
Measures of Disease Burden • Quality-Adjusted Life-Years (QALY): Measure of life expectancy corrected for loss of quality of life caused by diseases and disabilities. • One QALY is equal to 1 year of life in perfect health, and a year of less than perfect health is worth < 1 • Death is considered to be equivalent to 0
QALYs gained diagram This diagram shows two possible disease courses with two different interventions. Intervention B leads to greater utility (quality of life) and a longer life for the patient. This ‘area gained’ is thus QALYs gained.
Measures of Disease Burden • Disability Adjusted Life Years (DALY): Number of years lost due to ill-health, disability or early death. • It is an indicator of burden of disease. • DALY= YLD (years lived with disability) + YLL (years of life lost) Modified from DALY disability affected life year infographic.png
Proportional Age Distribution of Global DALYs In Females by Cause, 2011 (WHO, 2013)
Conclusions • Measuring disease frequency in a population is usually the first step in psychiatric epidemiology. • Three factors should be considered when measuring a disease occurrence in a population: (1) the number of affected individuals or cases; (2) the size of the population from which the cases arise; and, (3) the amount of time that this population is followed. • Before counting the number of affected cases, a case definition must be composedthat is usually based on physical and pathological examinations, diagnostic tests, and signs and symptoms.
Conclusions • Incidence and prevalence are the two basic measures of disease frequency. • Measures of disease burden includes crude and cause-specific mortality rates, and QUALYs and DALYs.
Helpful References • Aschengrau A, Seage G. Essentials of Epidemiology in Public Health. Second edition. Jones & Bartlett Learning, 2014. • Gordis L. Epidemiology. Fifth edition. Saunders; 2013. • Rothman KJ, Greenland S, Lash TL. Modern Epidemiology: Third edition. Lippincott Williams & Wilkins, 2012 • Penny Webb and Chris Bain. Essential Epidemiology. Essential Medical Texts for Students and Trainees. 2nd ed. Cambridge University Press, 2010. • SusserE, Schwartz S, Morabia A, Bromet EJ. Psychiatric Epidemiology: Searching for the causes of mental disorders. Oxford University Press, 2006 • Tsuang MT & Tohen M. Textbook in Psychiatric Epidemiology. 3rd. New York, Wiley-Liss, 2011.