Part II

4 Person, Place and Time

Learning Objectives

By the end of this chapter, the learner will be able to

  • Learn about the major subdivisions of epidemiology, descriptive and analytic.
  • Describe the importance of person, place, and time as the main variables used in descriptive epidemiology
  • Set the foundations for the understanding of new concepts that will be studied later, especially in the field of analytic epidemiology.
  • Appraise the importance of the concepts of person, place, and time as fundamental for the understanding of health phenomena and their related outcomes.

Person, Place, and Time
These three are considered the most common variables used in the study of epidemiology. Researchers in epidemiology and public health use these three variables (person, place, and time) to look for associations and health determinants that explain several health phenomena, especially illness. Frequently, it is important to know who the person is – although, it can be also an animal, insect, or any other living, or non-living thing who has been involved in a specific event, or, incident; and the place, and time in which this had occurred. It is kind of similar to what journalists use in the writing/reporting of their news, the who, where, and when. Who – the person, Where, the place, and When, the time. Although I had introduced the concepts of person, place, and time in a simplistic manner in the previous paragraph, the study of these three variables is not as simple as it appears, there are several sub-variables that are also used to make sense of the information, and to go into more details, is the main purpose of this chapter.


Characteristics of Person
When discussing who is at risk, the focus is typically on individuals. This is why it is commonly referred to as “the person.” However, in certain fields such as veterinary medicine or environmental health, the “who” may refer to an animal, ecosystem, lake, or coastal region. Despite this variation, the term “person” typically refers to humans. The characteristics of a person often align with the “individual lifestyle factors” category in the social determinants of health overview. An example of this can be seen in the image below.

Common characteristics of Person

Image from Health Knowledge
The most common subcategories of the variable person are race and ethnicity; gender, age, occupation, marital status, social economic status, and other socio-economic, and political variables.[1] Some of these characteristics may have a major impact on the health of a person more than others, but in general, all of them are important as it is a combination of factors (also called, determinants) that ‘determine’ the health of the individual. For example, no one can deny that socioeconomic conditions are extremely important in the life of a person, so, it is possible that more than one problem can be explained by this variable, but if every problem could be explained by only one indicator such as socioeconomic status, then, the measurement of this variable became confusing, or, not to say, useless; for this reason it is important to keep in mind that it is a combination of factors (variables, determinants) that explain the health status of the individual, the community, and the nation.


The category, or, variable gender has been frequently misused as an equivalent of sex, but sex is not the same as gender. Again, the National U.S. Census still uses the category sex to report, male, or female; but is essentially incorrect. People are either male or, female; but being male of female has more to do with biology because there is also a wide spectrum of variations as current research reports. See for example a recent image from an online publication that clearly shows that progress have been made on this area, especially the use of personal pronouns to refer to the gender category.[2]
Illustration of a group of diverse people with different pronouns written for
Image from Gender-identity

For reasons of conventionality, in epidemiology, the category gender is still referred as, sex. As an example of this category, it is recognized in public health and other fields of study that gender as a health determinant has a strong influence in the health status of women, especially in issues related to pregnancy and childbearing; it has been also reported that women tend to suffer more of series of chronic diseases that affect them differently compared to men.[3]


In a broad (general) manner, the variable age seems to be self-explanatory, since we know that people in general are children (with several subcategories categories), youth, adults, or, seniors (the preferred name these days is, older adults).

A person in different ages style Free Vector
Image from Freepik

But age is a complex variable especially for data collection, and analysis purposes, that is because age in a person defines several health stages. Epidemiology is concerned with the presence of disease for people in all ages, but the childhood and adulthood period are of such as concern because common knowledge says that are diseases that are characteristic of these two age periods. For example, immune preventable diseases such as polio, chicken pox, measles, etc., are common childhood diseases, and degenerative diseases especially those related to memory (such as dementia and similar neurological disorders) are more common in old age.

File:Childhood deaths from the five most lethal infectious diseases worldwide, OWID.svg
“Childhood deaths from the five most lethal infectious diseases worldwide”, Our World In Data, CC 3.0 Unported license.

There are several health issues that concern only to specific ages. The graph above shows the most common childhood diseases that are the major causes of childhood mortality in the world. While the next chart shows common diseases of the elderly population in the European region.

Image from From the World Health Organization (WHO)

A person’s occupation seems to be an obvious category because in general, people and their occupations are defined (in a broad way) by their jobs, or, work. But what about the people who work at home such as home makers, or, people who are unemployed, or, underemployed? This question raises the point that occupation is not as simple, and obvious as we generally think. If there is a place in which the variable occupation matters, is in the field of environmental sciences. Exposures to toxic chemicals, and other environmental contaminants are higher in certain occupations, or, jobs.[4]

Occupation during pandemic times in the U.S.

In the year 2020 affected by the COVID-19 pandemic, the rate of unemployment in the U.S. rose as a consequence of the complete disruption of people’s lives, and companies that went out of business due to the restrictions imposed by public health state and national ordinances.

Image from the Census Bureau.

See image:[5]

affects the health of individuals in different manners, but one area that is highly affected is mental health, and also the access to health care services, problems in paying rent, not to say the ability of buy food, and pay for other personal or, family needs. In the case of access to health services, there a very recent example is the case of women seeking sexual reproductive health services at publicly funded health clinics, according to insurance status before and during the COVID-19 pandemic. See image below:[6]

Unemployment in the United States before and during pandemic. Image from HealthAffairs

Marital status
In general, marital status more than any other variable is mainly a legal category or another example of a social construct. What defines a person as single or married is controversial if we think that two people living together for a long period of time are a couple, and although they may not be legally/officially married, they are in a married status. And it is for this reason that in some states in the U.S. after 15 years of living together, for legal purposes, a couple is considered married even if they are not officially married at the beginning of their relationship. For practical (operational) reasons in epidemiology, the marital status categories used in research are the U.S. National Census categories, which are four major categories: never married, married, widowed, and divorced.[7] Details of these mentioned categories are summarized in the table that follows:


Category Descriptor
“Married” Married category is divided into “married, spouse present,” “separated,” and “other married, spouse absent.” A person was classified as “married, spouse present” if the husband or wife was reported as a member of the household, even though he or she may have been temporarily absent on business or on vacation, visiting, in a hospital, etc., at the time of the enumeration [during the census data collection.].
“Separated” People reported as separated included those with legal separations, those living apart with intentions of obtaining a divorce, and other people permanently or temporarily separated because of marital discord.
“Other married” The group “other married, spouse absent” includes married people living apart because either the husband or wife was employed and living at a considerable distance from home, was serving away from home in the Armed Forces, had moved to another area, or had a different place of residence for any other reason except separation as defined above.
“Single” Single, when used as a marital status category, is the sum of never-married, widowed, and divorced people. “Single,” when used in the context of “single-parent family/household,” means only one parent is present in the home. The parent may be never-married, widowed, divorced, or married, spouse absent.

Table of Marital Status categories. Content prepared by the author using the U.S. Census Bureau marital status categories.[8]

Race and Ethnicity

Although it is accepted that race is a social construct. It has an impact on the lives of people more than any other, and it is one of the major factors used to explain health disparities, especially in the United States. The concepts of race and ethnicity have developed so much in recent years in this country. Theoretically, it has been known for decades that race and ethnicity are complementary categories, but not the same. Race is mostly the biological characteristics of the person, while ethnicity relates mostly to the ethnic group from which the person descends; both are socially constructed concepts. As the American Sociological Association defines, “Race” refers to physical differences that groups and cultures consider socially significant, while “ethnicity” refers to shared culture, such as language, ancestry, practices, and beliefs.[9]In this class, the classification used for epidemiological concepts, terms, statistical reasons (including categorization of ethnic groups in the U.S.), and calculations is, the Office Bureau of Census race classification[10]which essentially assigns the U.S. population to the following five categories:

The standards have five categories for data on race in the United States

American Indian or Alaska Native, Asian, Black or African American, Native Hawaiian or Other Pacific Islander, and White.

Although updates have been made in the 2021 census report, in general, these categories have been defined since 1997 [11], these basic definitions follow:

This table has been prepared using the U.S. Bureau of Census five categories used for data reporting on race and ethnicity. [12]
Description Category
 A person having origins in any of the original peoples of North and South America (including Central America), and who maintains tribal affiliation or community attachment. American Indian or Alaska Native
A person having origins in any of the original peoples of the Far East, Southeast Asia, or the Indian subcontinent including, for example, Cambodia, China, India, Japan, Korea, Malaysia, Pakistan, the Philippine Islands, Thailand, and Vietnam. Asian.
A person having origins in any of the black racial groups of Africa. Terms such as “Haitian” or “Negro” can be used in addition to “Black or African American.” Black or African American
A person of Cuban, Mexican, Puerto Rican, South or Central American, or other Spanish culture or origin, regardless of race. The term, “Spanish origin,” can be used in addition to “Hispanic or Latino.” Hispanic or Latino
A person having origins in any of the original peoples of Hawaii, Guam, Samoa, or other Pacific Islands. Native Hawaiian or Other Pacific Islander
A person having origins in any of the original peoples of Europe, the Middle East, or North Africa. White
In 2010, there was a change/addition to the five categories by creating the new, “Some Other Race” category, which includes all of the other responses (when a person responds to the Census form) not included in the five categories listed above (in the table). The person can respond with entries such as being, multiracial, mixed, interracial, or, Hispanic or belonging to a specific Latino group such as Mexican, Puerto Rican, Cuban, or Spanish).[13]

From the 2010 national census to now

From 2010 to 2020 lots have changed in the U.S. and the 2020 U.S. national census reflects these changes and acknowledges that the country has become more diverse than ever. To arrive at this conclusion, the Census Bureau used what they called, the ‘Diversity Index (DI),’ not discussed in detail here in this book, but this index should reflect the changes experienced by the country in recent years, and especially in the last decade.[14]


Religion in the health sciences and especially in public health is considered a protective factor[15], which means that religion (as a factor, or, state/condition) helps the individual and community to decrease the risk of developing negative health outcomes. For example, studies show that religion is a protective factor against drug use,[16] It is important to note here, that the article cited here, refer to ‘religiosity,’ and not to religion in general, which is an important distinction. Because it is not the religion itself, but the lifestyle associated with the religious practice. In this context, ‘religion‘ is considered a social determinant of health, but the question is, is it causal? More than causal, it is more of what is called, reversed causation and a series of confounding factors. Very limited studies (especially clinical trials) have been conducted to assess religion and its positive effect on the health of an individual. [17].

In a sense, it can be said that more than religion [as an entity itself], it is ‘spirituality’ that plays a role in the health of the individual, as an example taken from the field of psychology shows, see the visual below – there is a direct relationship between spirituality and health-related behaviors, and the same can be said, between spirituality and psychological well-being.[18]
Image from Frontiers in Psychology.

Socioeconomic status
Socioeconomic status is related to social class. In general, groups in society that have medium to high levels of income tend to have a better place in society compared to those who don’t, but from the public health perspective, the importance of socioeconomic status is a predictive factor of the lack of resources to stay healthy, access to health care services, and also access to healthy foods just to mention some of the essentials for a healthy life.[19]

Socioeconomic status Categories

For reasons of operationalization, this variable is usually divided into three levels (high, middle, and low), in this form researchers, and the public situate individuals, families, and communities (in the broad sense) in relation to others.[20]


On the other hand, for the mentioned levels of income (earnings), the Bureau of Census assigns a specific value using what is called, the Gini ratio (or index of income concentration) is a statistical measure of income equality ranging from 0 to 1. A measure of 1 indicates perfect inequality; i.e., one person has all the income, and the rest have none. A measure of 0 indicates perfect equality; i.e., all people have equal shares of income. The Census Bureau used grouped data to compute all Gini ratios. For a more detailed discussion, see Current Population Reports, Series P-60, No. 123.[21].


Since inequalities are found, the following graph (taken from the U.S. Census Bureau) reflects the income by gender in the U.S. in 2020. [22]

Image from the U.S. Census Bureau.

Characteristics of Place
As it was said at the beginning of this chapter, in epidemiology, the use of ‘Who?,’ ‘Where?,’ and ‘When?’ helps to remember that the ‘Who? is the person, the ‘Where’ refers to the place, and ‘When?’ The time.[23] In this section, the focus is on the ‘place’ or, the Where?


One of the major characteristics of place is that it could mean more than one thing, for example, place can refer to a location (an area, a city, a state, a country, etc.), but since place is a spatial concept, it is frequently described using geospatial coordinates such as latitude and longitude. That is one of the main reasons, in recent decades, geospatial analysis have grown so much in the field of epidemiology and public health in general. [24]. As an example, map of heart disease in the U.S. that was developed using GIS (or geographic information system) software is presented below:

Heart Disease Death Rates in the U.S. Image from CDC, Public Domain.

Since the image is developed using data that is represented by dots (the pixels in the image), another name for this type of image is, dot map, which was originally introduced (and probably created for the first time) by Dr. John Snow 1854 cholera epidemic, he drew the map without using any scale, because all he wanted was to designated where the major sources of infection where coming from in the city of London. See this map below:

Dr. Snow’s map showing cholera cases in London during the epidemic of 1854. Image from Wikipedia, Public Domain.
Common characteristics of place


The definitions of urban and rural create some confusion, what is rural? And when does rural end and urban begin? This has been a growing discussion in recent years since most urban areas in the United States have grown so much that rural areas have been engulfed by the metro area. The U.S. Bureau of Census classifies communities as urban and rural by relating them to Metropolitan Statistical Areas (MSAs) and census tracts.

In this context, it is customary in epidemiology and public health to use the NCHS urban-rural classic classification that uses six levels (of categories) as follow: [25]

Category Description
Large metro, central the entire population of the largest principal city of the MSA*
Large metro, fringe Counties located in an MSA and with 1 million or more population
Medium metro Population of 250,000 – 999,999 in an MSA
Small metro Population of 50,000 – 249,999 in an MSA
Nonmetropolitan Micropolitan (in an MSA)
Nonmetropolitan Noncore (not in an MSA)
* MSA = Metropolitan Statistical Area. The MSAs are defined by the OMB (Office of Management and Budget) as those places having at least one urbanized area with a minimum population of 50,000. ** Census tracts = small geographic subdivisions of cities, counties, and adjacent areas. the average tract has a population of 4,000 residents and is designated to provide a degree of uniformity of population economic status and living conditions within each tract.

The health of urban versus rural communities

Overall, the differences between urban and rural populations are probably not as dramatically different, however, health differences have been reported between rural and urban communities. These differences between urban and rural populations are probably not as dramatically different, but people living in rural areas are more likely to engage in negative behaviors that affect their health and quality of life. [26]


Differences in morbidity and mortality have been observed among urban and rural populations in the United States. Some authors recently reported the trend one of the major disparities between urban and rural is in the issue of mortality, which persisted over time (the authors analyzed 47 years of data related to the problem, and they found that the high level of mortality is tied to poverty more than any other associated factor.[27]. A general outline of the differences between urban and rural health is presented in the image below:[28].
The Difference between Rural and Urban Areas | FDB
Image from Difference Between

To illustrate in a more graphic manner, I found this infographic that is very useful.[29]

The Challenge of Rural Health Care
Image from IEEE Pulse.

Variations of disease and other health outcomes within the country

The variation of disease and related health outcomes have been observed also within the same country’s geographical regions. For example, in the United States comparisons of disease rates have been established by region (Pacific, Central, Mountain, and Atlantic), and by state. In the case of some chronic diseases such as cancer, it varies across states.
Lyme Disease Maps: Historical Data | Lyme Disease | CDC
Image from CDC, Lyme Disease Historical DataPublic Domain.


[30] Similar situation has been reported on multiple sclerosis. Significant variations of multiple sclerosis rates have been found between north and south of the United States.[31] Also, similar variations can be found in the case of infectious diseases.[32]. A typical example of this is observed in the rates of Lyme disease in the U.S. in which the most affected areas are represented by the Midwest states and the northern East coast. [33]

Variations of disease and other health outcomes across countries

International variations across countries show significant differences and some similarities such as the presence of cardiovascular diseases that can be found around the world. In other cases, the differences are significant in terms of mortality, for example in the cases of the current COVID-19 pandemic, the differences across countries in the world varied, especially in those countries with poor healthcare systems, high levels of poverty, and poor public health infrastructure among other factors. These mentioned conditions are expected to be found mainly in developing countries; however, the U.S. is not far away from those countries as a recent report from The Commonwealth Fund has found. This report shows the U.S. healthcare system is generally weak with an inefficient administrative system among other indicators (see table below). Here is the healthcare system performance of countries with similar levels of development and its rankings during the COVID-19 global pandemic.[34]
OVERALL RANKING 3 10 8 5 2 6 1 7 9 4 1
Access to Care 8 9 7 3 1 5 2 6 10 4 11
Care Process 6 4 10 9 3 1 8 11 7 5 2
Administrative Efficiency 2 7 6 9 8 3 1 5 10 4 11
Equity 1 10 7 2 5 9 8 6 3 4 11
Health Care Outcomes 1 10 6 7 4 8 2 5 3 9 11
Image (table above) taken (with formatting modifications) “Health Care System Performance Rankings,” from the Commonwealth Fund. Note: the score used in this study was developed using a ranking of 1-11, but there are more details in the analysis of it.
Without going into details about each one of the criteria used by the authors of this publication, the U.S. is an outlier, it performed badly, and its overall performance falls well below the average for the rest of the developed countries listed in the report. The U.S. is last on all domains of performance except care process, on which it ranks second (or #2).[35]

Other factors associated with variation in disease

As the examples above have shown, the differences in the detection of disease are mainly linked to location and geography. Other factors that may help to explain variations by place may be the tendency of some ethnic groups to live in some specific areas of the United States, and around the world. For example, Mormons tend to have a more frugal lifestyle than people living in Vegas, which accounts for variations of disease. Another factor could be social economic conditions and poverty levels found in some states in the country, for example, the high number of homeless persons in San Francisco, California; and New York City. Also, the levels of poverty in some states can explain the high levels of mental health problems found in very impoverished populations.

Characteristics of Time

Following the who, where and when that we used in previous section of the chapter, time is the when, when did it happened? Adding the time dimension to the report of health phenomena (or, problems) makes a lot of sense since there are events in which the time is enough to bring the event to mind, for example, COVID-19 pandemic time, immediately the year 2020-21 comes to mind.

A graphic representation of the relationship among time and disease. Image from apsnet.

So, if we know who (person) is affected, where (location) did this happened, and when, which is time; then, a picture of the health event is completed.[36]. In terms of a graphical representation of the concept of time, statistically speaking, the variable time belongs to the ‘X’ coordinate and the ‘disease’ to the ‘Y,’ as the image above represent.

Other time concepts
In addition, it is customary (in most epidemiology books) to include in this section about time, the main characteristics of time, which includes the concepts of cyclic variations, point epidemics, secular trends, and clustering. All of these categories will be discussed in the content that follows.

Cyclic Fluctuations (or, cyclic variations)

What are cyclic fluctuations? Taking the term directly from the Concise Encyclopedia of Statistics, “Cyclical fluctuations is a term used to describe oscillations that occur over long periods about the secular trend line or curve of a time series.”[37] In other words, increases and decreases in the frequency of diseases and health condition over a period of years or within each year. Although it is not infrequent that these fluctuations (or, oscillations) reflect seasonal trends, they are not the same. So, how to identify cyclic fluctuations versus seasonal trends? Let’s start by looking at the seasonal factors, these can be identified by winter and summer; the day of the week, the month or, the quarter of the year. So, for quantification purposes, seasonal health events are always reported in a fixed and known period. And, the periods are used to defined the time series which are the same as seasonal series. On the other hand, cyclic fluctuations appears in for example short period such as 2 years, and these fluctuations (increases and decreases, or, rise and falls) are of not a fixed period.[38]

A common example of a seasonal disease is flu
which usually appears in the months of winter and early spring. most of the time flu activity peaks between December and February, but activity can last as late as May.[39] See image below:

Flu Peak Activity chart
United States flu season. From CDC. Image licensed CC SA.

On the other hand, cyclic fluctuations frequently involved a disease that it does not appear (frequency) in specific times but appear and reappear in certain periods of time, for example, Pertussis (whooping cough), which peaks in disease every 3 to 5 years. [40]. See graph below:

Image: Reported pertussis incidence by age group: 1990-2018. From CDC. Image licensed CC SA. The data for this graph can be found at CDC.

Secular trends

The world secular comes from the Latin, “Saeculum”, that in general terms mean, someone who is not from the clergy, but in epidemiology, secular is used to refer to long period of times (usually years) in the occurring of some diseases. This trend is influenced by the degree of immunity in the population and possibly nonspecific factors such as poverty levels, or, lack of access to preventive health services.[41] An example is salmonellosis, see graph below:

Figure 9-2. An example of a disease showing a seasonal trend.
Reported human Salmonella isolations, by 4-week average, in the United States from 1968 to 1980. Source of the Image license not stated.


Cases about a disease can cluster in a group of individuals, closely grouped in time and place.[42] This can happen with diseases such as cancer. This cluster may have been linked to environmental exposures.[43] An image showing for example cancer clusters by state is shown below:

Geographical clusters of U.S. counties with significant high or low... | Download Scientific Diagram
Geographical clusters of US counties with significant high or low breast-cancer. Source, Licensed CC BY 2.0

Note: There are more content that can be covered under the topic discussed in this chapter, but for now, the book will cover only what it has been written until here.


This chapter has introduced the concepts of person, place and time commonly used in epidemiology. The study of these three categories is not as simple as it appears, there are several sub-categories that exist for each one of the main topics, and it is the intention of the author that the images, and examples helped to make sense of the information.

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