Introduction to Methods in Psychiatric Epidemiology

Matias Irarrazaval MD MPHUniversity 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 Measures existing cases of disease and is expressed as a proportion

Incidence Measures new cases of disease and is expressed in person-time units

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

The proportion of individuals with the condition at a specified point in time.

Period prevalence

The proportion of individuals with the condition at any time during a specified time period or interval.

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=

Prevalence in June 2007: 2/10 = 20%

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

Is related to a measure of the population at risk during the study period.Is measured in person-time units.

Incidence risk (cumulative incidence)

Is related to the population at risk at the beginning of the study period.

• 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 Rate

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: 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 Risk (Cumulative Incidence)

Incidence Number of people who develop disease in a specific time period

Risk Number of disease-free persons at beginning of that time period=

• 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.

Measures of Disease Burden

• 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%

Mortality and Morbidity

• 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

Measures of Disease Burden

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. 

Modified from DALY disability affected life year infographic.pngModified from DALY disability affected life year infographic.png

• 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)

Measures of Disease Burden

Proportional Age Distribution of Global DALYsIn 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 composed that is usually based on physical and pathological examinations, diagnostic tests, and signs and symptoms.

• 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.

Conclusions

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.

• Susser E, 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.