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01_concepts-and-measures.Rmd
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01_concepts-and-measures.Rmd
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# Concepts and measures {#concepts-and-measures}
```{r setup, warning=FALSE, message=FALSE, echo=FALSE, results='hide'}
library(DT)
library(ggplot2)
library(keyring)
library(knitr)
library(lubridate)
library(magrittr)
library(plotly)
library(purrr)
library(readr)
library(rlang)
library(scales)
library(slickR)
library(stringr)
library(svglite)
library(tibble)
library(tidycensus)
library(tidyr)
library(timevis)
```
:::{.rmdnote}
## Learning objectives {.unnumbered}
* Learn what demography is and why it's important
* Know how to specify a population in demographic terms
* Apply the balancing equation and its main components to track change in population size over time
* Learn what person-periods are and how to approximate them
* Use occurrences, person-periods, and observations to construct demographic rates and probabilities
* Understand the differences between rates and probabilities
* Learn the relationships and differences between crude, instantaneous, and mean annualized growth rates
* Learn the differences between a period and a cohort, and their relation to rates and probabilities
:::
## What is demography and why is it important?
:::{.rmdnote}
### Definitions of demography {.unnumbered}
* "...the scientific study of human **populations** primarily with respect to their **size**, their **structure** and their **development**; it takes into account the quantitative aspects of their general characteristics. [@population_multilingual_1958]
* The study of human **populations** in relation to the **changes** brought about by the interplay of **births**, **deaths**, and **migration**. [@pressat1985dictionary]
* "...the study of of human **populations** -- their **size**, **composition** and **distribution** across **space** -- and the **process** through which populations **change**. [@suda_demography]
*Emphasis added.*
:::
**Discussion questions**
<details>
<summary>What are the "processes" that the Stockholm University Demographic Unit's definition drives at? **Tap for answer**</summary>
* **Birth:** Entering the population from the womb
* **Death:** Exiting the population because we're all mortal
* **Migration:** Physically moving in our out of the population's location
</details><br>
<details>
<summary>What are three basic dimensions along which these changes occur? **Tap for answer**</summary>
* **Time:** For example, you can track the number of deaths from year to year
* **Space:** You can track how a population moves its location over time, or how individuals move into our out of a local population over time.
* **Structure:** You can disaggregate a population into subpopulations. For example, by age, by race or ethnicity, by religion, by gender. This structure may change over time.
</details>
<br>
### Okay so what's a "population" then? {.unnumbered}
**For statistians:** A collection of items
**For demographers:**
1. A **collection of persons**...
1. who **meet certain criteria**
1. alive at a **specified point in time**...
<br><br>
<details>
<summary>Anything odd about point 1 above? **Tap for answer**</summary>
Non-human biologists do demography, too.
* Dr. Hal Caswell, Professor of Mathematical **Demography** at University of Amsterdam: https://www.uva.nl/en/profile/c/a/h.caswell/h.caswell.html
* Wrote influential book on matrix **population models** (aka... **demography**)
> "My research focuses on **population models**, usually based on matrices, for **plants, [non-human] animals, and humans**. I am interested in stochastic processes in **demography**..."
>
> -- Hal Caswell (my emphasis)
<img width="25%" src="images/hal-caswell.jpg" alt="Hal Caswell">
<img width="25%" src="images/hal-caswell-matrix-population-models.jpg">
</details>
<br>
<details>
<summary>Let's think of some examples. **Tap for answer**</summary>
* **Collection:** People...
* **Criteria:** living in King County, Washington...
* **Specified point in time:** on April 1, 2019
```{r king_acs1_totpop_05_19, echo=FALSE, warning=FALSE, message=FALSE, cache=TRUE}
get_king_acs1_totpop <- function(year) {
tidycensus::get_acs(
year = year,
geography = "county",
variables = "B01003_001",
state = "Washington",
county = "King County",
survey = "acs1"
)
}
years <- 2005:2019
king_acs1_totpop_05_19 <- years %>%
purrr::map(function(y) get_king_acs1_totpop(y)$estimate) %>%
as.numeric() %>%
tibble::tibble(
year = years,
acs1_totpop_estimate = .
)
king_acs1_totpop_05_19_endpoints <-king_acs1_totpop_05_19 %>%
dplyr::filter(year %in% c(2005, 2019))
king_acs1_totpop_05_19_growth <- king_acs1_totpop_05_19_endpoints %>%
dplyr::mutate(
growth_14_year = (acs1_totpop_estimate
/ dplyr::lag(acs1_totpop_estimate)) - 1) %>%
dplyr::filter(!is.na(growth_14_year)) %>%
dplyr::pull(growth_14_year) %>%
scales::percent(., accuracy = 1L)
```
```{r king_acs1_totpop_19, echo=FALSE, warning=FALSE, message=FALSE}
king_acs1_totpop_19 <- king_acs1_totpop_05_19 %>%
dplyr::filter(year == 2019) %>%
dplyr::pull(acs1_totpop_estimate) %>%
magrittr::divide_by(1000000) %>%
round(1)
```
<center>**`r paste(king_acs1_totpop_19, "M people")`**^[1-year American Community Survey 2019 total population estimate]</center>
</details>
<br>
**Also for demographers:**
<center>_Population as an **enduring** collection of individuals_</center>
By "enduring", PHG mean those characteristics of a population that don't change.
<br>
<details>
<summary>Extending our Seattle metro example to "enduring" collections **Tap for answer**</summary>
Now we can see how the population changes, in this case over time...
King County population grew by `r king_acs1_totpop_05_19_growth` over 14 years.
```{r king_acs1_totpop_05_19_plot, echo=FALSE, warning=FALSE, message=FALSE, fig.alt="One-year ACS total population estimates - King County, WA (2005 - 2019)"}
plot_king_acs1_totpop_05_19 <- king_acs1_totpop_05_19 %>%
ggplot2::ggplot(aes(x = year, y = acs1_totpop_estimate)) +
ggplot2::geom_point() +
ggplot2::geom_line() +
ggplot2::geom_text(
data = king_acs1_totpop_05_19_endpoints,
aes(x = year + 0.6,
label = acs1_totpop_estimate %>%
scales::unit_format(unit = "M",
scale = 1e-6,
accuracy = 0.1,
sep = "")(.))
) +
ggplot2::scale_x_continuous(breaks = years) +
ggplot2::scale_y_continuous(
labels = scales::label_number(suffix = "M", scale = 1e-6, accuracy = 0.1),
) +
ggplot2::labs(
title = "Total population - King County, WA (2005-2019)",
subtitle = "1-year American Community Survey estimates"
) +
ggplot2::xlab(NULL) +
ggplot2::ylab(NULL) +
ggplot2::theme_minimal() +
ggplot2::theme(panel.grid.minor.x = element_blank())
plot_king_acs1_totpop_05_19
```
And here is a more interactive example from Italy made by [Eddie Hunsinger](https://www.linkedin.com/in/eddiehunsinger/): [https://shiny.demog.berkeley.edu/eddieh/lx_ndx_Italy/]{target="_blank"}, which we'll return to in week 3
</details>
<br>
## The balancing equation of population change
* Consider an observation period of length $T$
* For now, arbitrarily set the period's starting point at time $t = 0$
$\begin{align}
N(T) &= \textsf{ (Ending population size at time } T \textsf{)} \\
&+ N(0) \textsf{ (Starting population size at time } 0 \textsf{)} \\
&+ B[0,T] \textsf{ (Number of births from start to end)} \\
&- D[0,T] \textsf{ (Number of deaths from start to end)} \\
&+ I[0,T] \textsf{ (Number in-migrations from start to end)} \\
&- O[0,T] \textsf{ (Number out-migrations from start to end)} \\
\end{align}$
<details>
<summary>Organized by ways to enter vs. exit a population... **Tap for answer**</summary>
$\begin{align}
N(T) &= N(0) \\
&+ B[0,T] + I[0,T] \textsf{ (Ways to enter)} \\
&- D[0,T] - O[0,T] \textsf{ (Ways to exit)}
\end{align}$
</details>
<br>
<details>
<summary>Organized by natural increase vs. net migration... **Tap for answer**</summary>
$\begin{align}
NI[0,T] &= B[0,T] - D[0,T] \textsf{ (Natural increase)} \\
NM[0,T] &= I[0,T] - O[0,T] \textsf{ (Net migration)}
\end{align}$
</details>
<br>
<details><summary>And putting it all together... **Tap for answer**</summary>
$N(T) = N[0] + NI[0,T] + NM[0,T]$
</details>
<br>
### Balancing equation as flows and stocks {.unnumbered}
* Boxes represent states that individuals in a population can be in
* Arrows represent a flow of individuals from one state to another
![Balancing equation stock flow chart](./images/balancing-equation-stock-flow.svg){width=100%}
### Balancing equation example: Sweden in 1988 {.unnumbered}
From PHG pg. 9 Box 1.2
```{r swedish_waterfall, echo=FALSE, warning=FALSE, message=FALSE}
measures <- c(
"Starting population",
"Births",
"Deaths",
"In-migrations",
"Out-migrations",
"Ending population"
)
notations <- c(
"N(1988)",
"B[1988,1989]",
"D[1988,1989]",
"I[1988,1989]",
"O[1988,1989]",
"N(1989)"
)
values <- c(8416599, 112080, -96756, 51092, -21461) %>%
c(., sum(.)) %>%
abs(.) %>%
format(., big.mark = ",", trim = TRUE) %>%
paste(c("", rep(c("+ ", "\u2013 "), 2), "= "), ., sep = "")
enters_exits = c(NA_character_, rep(c("Enters", "Exits"), 2), NA_character_)
ni_nm = c(
NA_character_,
"Positive impact on NI[0,T]",
"Negative impact on NI[0,T]",
"Positive impact on NM[0,T]",
"Negative impact on NM[0,T]",
NA_character_
)
swedish_waterfall <- tibble::tibble(
measure = measures,
notation = notations,
value = values,
enters_exits = enters_exits,
ni_nm = ni_nm
)
DT::datatable(
swedish_waterfall,
options = list(ordering = FALSE, dom = "t"),
rownames = FALSE,
colnames = rep("", 5)
) %>%
DT::formatStyle(columns = 1, fontWeight = "bold") %>%
DT::formatStyle(
columns = 2, fontStyle = "italic", fontFamily = "MathJax TeX"
) %>%
DT::formatStyle(
columns = 4,
color = styleEqual(
levels = c("Enters", "Exits"),
values = c("skyblue", "orange")
)
) %>%
DT::formatStyle(
columns = 5,
backgroundColor = styleEqual(
levels = c(
"Positive impact on NI[0,T]",
"Negative impact on NI[0,T]",
"Positive impact on NM[0,T]",
"Negative impact on NM[0,T]"
),
values = rep(c("skyblue", "orange"), 2)
)
)
```
:::{.rmdtip}
**DEMOGRAPHY & DATA SCIENCE**
### Balancing equation analogy: A company's employees {#balancing-data-science .unnumbered}
Let's apply this lesson to a population a data scientist might work with:
<center>**A company's employee headcount grows**</center>
Think of analogies to the components of the balancing equation
<br><br>
<details>
<summary>Analogy to births $B[0,T]$? **Tap for answer**</summary>
New hires, BUT...
* Thinking about how birth vs. hiring happen, what's a weaknesses of this analogy?
* Thinking about how some new hires worked at the company before, what's another weakness of the analogy?
</details>
<br>
<details>
<summary>Analogies to deaths $D[0,T]$? **Tap for answer**</summary>
All-cause terminations, BUT...
* Thinking about the state "Death" below, what's a potential weakness of this analogy?
* Where could (at least some terminations) flow instead?
![Balancing equation stock and flow chart again](./images/balancing-equation-stock-flow.svg){width=100%}
</details>
<br>
<details>
<summary>Analogies to in-migrations $I[0,T]$ and out-migrations $O[0,T]$? **Tap for answer**</summary>
If the population is defined as employees at the company:
* Terminations who remain in the workforce (out-migration)
* Re-hires (in-migration)
* Hires from other companies (in-migration again)
If the population is defined as a subset of employees at company:
* Transfers into and out of departments, teams, job functions, etc.
</details>
<br>
```{r employee_data_setup, echo=FALSE, message=FALSE, warning=FALSE}
# Load employee data from GitHub
source_url <- paste0("https://raw.githubusercontent.com/",
"teuschb/hr_data/master/datasets/",
"turnover_babushkin.csv")
employees <- readr::read_csv(url(source_url), col_types = cols())
# Compute annualized period attrition rate for the full period and for all
# possible combinations of the categorical or integer value columns
## Write a function to compute annualized attrition rate for grouped or
## ungrouped data
compute_annualized_attrition_rate <- function(.data,
employee_months_var,
terminations_var) {
.data %>%
dplyr::summarize(employee_months = sum({{employee_months_var}}),
number_terminations = sum({{terminations_var}})) %>%
dplyr::ungroup() %>%
dplyr::mutate(employee_years = employee_months / 12,
annualized_attrition_rate = (number_terminations
/ employee_years))
}
compute_easy_annualized_attrition_rate <- function(.data) {
if (any(names(.data) == "tenure")) {
.data %>%
compute_annualized_attrition_rate(
employee_months_var = tenure,
terminations_var = left_company
)
} else {
.data %>%
compute_annualized_attrition_rate(
employee_months_var = employee_months,
terminations_var = number_terminations
)
}
}
## Use the function to complete the task
attrition_most_vars <- employees %>%
dplyr::group_by(gender,
age,
industry,
manager_gender,
commute_type) %>%
compute_easy_annualized_attrition_rate()
# Because you have access to exact person-years, you can easily compute
# annualized attrition for any arbitrary combination of variables...
## ... either by aggregating the microdata in a different way:
attrition_gender <- employees %>%
dplyr::group_by(gender) %>%
compute_easy_annualized_attrition_rate()
## ... or from pre-aggregated data
attrition_all_up <- attrition_most_vars %>%
compute_easy_annualized_attrition_rate()
```
**Example:** Below is random sample from a data table of employees from a real Russian company^[Documented here: https://www.kaggle.com/davinwijaya/employee-turnover]:
* **tenure** is the number of months the employee worked at the company
* **left_company** equal 1 if the employee terminated, 0 otherwise
* Notice the other attributes available like **gender** and **age**
```{r employee_datatable, message=FALSE, warning=FALSE, echo=FALSE}
employee_selection <- employees %>%
dplyr::select(tenure,
left_company,
gender,
age,
industry,
commute_type,
manager_gender) %>%
dplyr::sample_n(nrow(.))
DT::datatable(
employee_selection,
options = list(scrollX = TRUE, ordering = FALSE)
)
```
From this data, we can easily compute number of total terminations as `r attrition_all_up$number_terminations`.
We can also disaggregate termination counts by variables, such as....
**Attrition by gender**
* Gender definition and category names aren't inclusive at this employer
* Looks like more women ("f") than men ("m") left the company
<details>
<summary>What information is missing if we want to compare the pace of termination by gender? **Tap for answer**</summary>
* Number of employees at risk of leaving the company
* How long those employees were at risk of leaving
</details>
```{r attrition_gender, echo=FALSE, message=FALSE, warning=FALSE}
attrition_gender_select <- attrition_gender %>%
dplyr::select(gender, number_terminations)
DT::datatable(
attrition_gender_select,
rownames = FALSE,
colnames = c("Gender", "Number of terminations"),
options = list(ordering = FALSE, dom = "t"),
width = "50%"
)
```
<br><br>
That brings us to the topic of demographic rates...
:::
## The structure of demographic rates
For demographers...
$$
\textsf{Rate} =
\frac{\textsf{Number of occurrences of an event of interest}}
{\textsf{Person-periods of exposure to the risk of occurrence}}
$$
:::{.rmdimportant}
**KEY CONCEPT**
**Person-periods** (e.g., person-years) are the sum across a population of all the time that individuals were exposed to the risk of some event.
:::
<details>
<summary>From PHG, what type of rate is this? **Hint:** Look in the denominator above! **Tap for answer**</summary>
* Occurrence rate, or...
* Exposure rate
</details>
<br>
<details>
<summary>The book's definition uses "person-years." I used "person-periods." Why? **Tap for answer**</summary>
* Most traditional demographic rates are annual. Why might that be?
* In some cases, period length longer or shorter than a calendar year is more appropriate. Example?
</details>
<br>
:::{.rmdimportant}
**KEY CONCEPT**
A ratio ain't a(n occurrence aka exposure) rate!
**Example:** The U.S. monthly unemployment "rate" ([U-3](https://www.bls.gov/news.release/empsit.t15.htm)) is defined as:
$$\frac{\textsf{Count of the unemployed from Current Population Survey (CPS)}}
{\textsf{Count of the employed plus unemployed from CPS}}$$
<details>
<summary>What about the numerator makes this not a demographic rate? **Tap for answer**</summary>
* It isn't a count of occurrences
* Instead, it's a count of people at a point in time
* Later, we'll see that such counts are an estimate of monthly person-periods
</details>
<br>
<details>
<summary>What about the denominator makes this a funky unemployment "rate"? **Tap for answer**</summary>
Unemployed people aren't at risk of becoming unemployed
</details>
<br>
<details>
<summary>What could we change to make it a rate? **Tap for answer**</summary>
* Make the denominator a count of employed person-periods
* Make the numerator a count of transitions from employment to unemployment
</details>
:::
### Person-periods: A central concept in demography {.unnumbered #person-periods}
<table width="100%"><tr>
<td>Let's illustrate with a lifeline of the life of Catherine the Great, Empress of Russia
</td>
<td>
<img src="images/catherine-the-great-smithsonian-magazine.png" width = 20%" alt="Catherine the Great and The Great">
</td>
</tr></table>
```{r catherine_the_great_lifeline, echo=FALSE, warning=FALSE, message=FALSE}
ctg_events <- tibble::tribble(
~content, ~start, ~end,
"Born", "1729-05-02", NA,
"Married Peter III", "1745-07-17", NA,
"1st miscarriage", "1752-12-20", NA,
"2nd miscarriage", "1753-6-30", NA,
"Birthed Paul I", "1754-10-01", NA,
"Birthed Anna", "1757-03-08", NA,
"Birthed Alexei", "1762-04-11", NA,
"(Maybe) birthed Elizabeth", "1775-05-25", NA,
"Died", "1796-11-17", NA
) %>%
dplyr::mutate(
content =
paste(content,
lubridate::year(start),
lubridate::year(end)) %>%
stringr::str_replace_all(pattern = " NA", "")
)
ctg_reproductive_years <- ctg_events %>%
dplyr::filter(stringr::str_detect(content, "^Born")) %>%
dplyr::pull(start) %>%
as.Date() %>%
tibble::tibble(
content = "Reproductive years",
start = . %m+% lubridate::years(15),
end = . %m+% lubridate::years(50)
) %>%
dplyr::mutate(dplyr::across(start:end, as.character))
ctg_reign <- tibble::tibble(
content = "Reign as Empress of Russia",
start = "1762-07-09",
end = "1796-11-17"
)
dplyr::bind_rows(
ctg_events,
ctg_reproductive_years,
ctg_reign
) %>%
dplyr::mutate(id = dplyr::row_number()) %>%
timevis::timevis()
```
### From lifelines to event counts and person-periods {.unnumbered}
Basic facts
* Consider a group (population?) of individuals denoted $G$
* $A_i$: Beginning of lifeline of individual $i \in G$
* $B_i$: End of individual $i$'s lifeline
* $\theta_{ij}$: The $j$^th^ among $N_i$ occurrences in the lifeline of individual $i$
* $T_i = B_i - A_i$: The length of individual $i$'s lifeline
<details>
<summary>What's another demographic term for $T_i$? **Tap for answer**</summary>
**PERSON-PERIODS!**
</details>
<br>
Rate for the group defined over their entire lifelines:
$$\textsf{Rate}_G = \frac{\sum_{i \in G} N_i}
{\sum_{i \in G} T_i}$$
A toy example to illustrate how exposure rates work...
```{r lifelines_plot, echo=FALSE, message=FALSE, warning=FALSE}
lifelines <- tibble::tibble(
lifeline_vertical_position = 4:1,
start = c(4, 5, 5, 6),
end = c(20, 16, 16, 18)
) %>%
dplyr::mutate(person_periods = end - start,
i = 5 - lifeline_vertical_position,
start_label = paste0("A[", i, "]"),
end_label = paste0("B[", i, "]"),
person_periods_label_position = start + person_periods / 2,
person_period_label = paste0("T[", i, "] == ", person_periods))
occurrences <- tibble::tibble(i = c(rep(1, 3), 2, 4),
occurrence_time = c(6, 7, 14, 14, 12)) %>%
dplyr::group_by(i) %>%
dplyr::mutate(j = dplyr::row_number()) %>%
dplyr::ungroup() %>%
dplyr::mutate(occurrence_label = paste0("theta[", i, j, "]"))
joined_data <- dplyr::left_join(lifelines, occurrences)
total_occurrences <- nrow(occurrences)
total_person_periods <- sum(lifelines$person_periods)
lifelines_plot <- ggplot2::ggplot(joined_data) +
ggplot2::aes(x = start, xend = end,
y = lifeline_vertical_position,
yend = ..y..) +
ggplot2::geom_segment() +
ggplot2::geom_label(aes(label = start_label), parse = TRUE) +
ggplot2::geom_label(fill = "lightgray",
parse = TRUE,
aes(x = end, label = end_label)) +
ggplot2::geom_text(vjust = -1,
parse = TRUE,
aes(x = person_periods_label_position,
label = person_period_label)) +
ggplot2::geom_point(aes(x = occurrence_time)) +
ggplot2::geom_text(vjust = 1.1,
parse = TRUE,
aes(x = occurrence_time, label = occurrence_label)) +
ggplot2::ylim(0, 5) +
ggplot2::xlab("Time") +
ggplot2::ylab(NULL) +
ggplot2::theme_minimal() +
ggplot2::theme(
panel.grid = element_blank(),
axis.text = element_blank(),
axis.line.x = element_line(arrow = arrow(angle = 30, type = "closed"))
)
lifelines_plot
```
<details><summary>How many **occurrences** of event $\theta$ in this picture? **Tap for answer**</summary>
`r total_occurrences`
</details>
<br>
<details><summary>How many **person-periods**? **Tap for answer**</summary>
`r total_person_periods`
</details>
<br>
<details><summary>What is the **life-time rate**? **Tap for answer**</summary>
`r total_occurrences` $\div$ `r total_person_periods` = `r scales::percent(total_occurrences / total_person_periods, accuracy = 1L)`
</details>
<br>
:::{.rmdimportant}
**KEY CONCEPT**
Exposure rates weight individuals in the denominator by the number of person-periods they were exposed to the risk of the event.
:::
## Period rates and person-years (er... person-periods)
**Period rate**: A rate that limits occurrence and exposure time to those experienced by a population during a specific period of time:
$$
\textsf{Rate}[0,T] =
\frac{
\textsf{Number of occurrences between time } 0 \textsf{ and } T
}
{
\textsf{Number of person-periods lived between time } 0 \textsf{ and } T
}
$$
:::{.rmdimportant}
**KEY CONCEPT**
People can live fractional (i.e., less than one) person-periods!
```{r exact_period_years_eoq, echo=FALSE, message=FALSE, warning=FALSE}
grades_day <- as.Date("2022-03-23")
current_year <- lubridate::year(grades_day)
grades_day_display <-
paste0(month.name[lubridate::month(grades_day)], " ",
lubridate::day(grades_day), ", ",
current_year)
first_day_current_year <-
paste(current_year, "01", "01", sep = "-") %>%
as.Date()
first_day_next_year <-
paste(current_year + 1, "01", "01", sep = "-") %>%
as.Date()
total_days_current_year <-
as.numeric(first_day_next_year - first_day_current_year)
person_days_ytd <-
as.numeric(grades_day - first_day_current_year)
person_years_ytd <- person_days_ytd / total_days_current_year
person_years_ytd_display <- scales::percent(person_years_ytd, accuracy = 0.1)
```
**Example:** By the time you get your final grade for this course on `r grades_day_display`, you'll have lived `r person_days_ytd` person-days in `r current_year` so far, which is `r person_years_ytd_display` of a person-year...
... unless, of course, you were born this year! `r emo::ji("laughing")`
Either way, congrats. `r emo::ji("celebrate")`
:::
Let's illustrate with a toy example:
* A population of 7 people
* Observed over 1 calendar year...
First, let's look at the lifelines of each individual in the population...
```{r period_person_years_plot_setup, echo=FALSE, message=FALSE, warning=FALSE}
individual_person_quarters <- tibble::tribble(
~id, ~start, ~person_quarters,
1, 0.00, c(1, 1, 1, 1),
2, 0.00, c(1, 1, 1, 0),
3, 0.25, c(0, 1, 1, 1),
4, 0.00, c(1, 1, 1, 1),
5, 0.00, c(1, 1, 0, 0),
6, 0.25, c(0, 1, 1, 0),
7, 0.75, c(0, 0, 0, 1)
) %>%
dplyr::mutate(y = 8 - id) %>%
tidyr::unnest(cols = c(person_quarters)) %>%
dplyr::mutate(person_years = person_quarters * 0.25) %>%
dplyr::group_by(id) %>%
dplyr::mutate(quarter_number = dplyr::row_number())
individual_person_years <- individual_person_quarters %>%
dplyr::group_by(id, y, start) %>%
dplyr::summarize(person_years = sum(person_years)) %>%
dplyr::ungroup() %>%
dplyr::mutate(end = start + person_years,
person_years_label = paste(person_years, "person-years"),
person_years_label_x = start + person_years / 2)
person_years_by_quarter <- individual_person_quarters %>%
dplyr::group_by(quarter_number) %>%
dplyr::summarize(person_years = sum(person_years)) %>%
dplyr::ungroup() %>%
dplyr::mutate(quarter_name = paste0("Q", quarter_number),
person_years_label = paste(person_years, "person-years"))
total_person_years <- sum(person_years_by_quarter$person_years)
individual_person_years_plot <- individual_person_years %>%
ggplot2::ggplot() +
ggplot2::aes(x = start, xend = end, y = y, yend = ..y..) +
ggplot2::geom_segment() +
ggplot2::geom_point(aes(x = start)) +
ggplot2::geom_point(aes(x = end)) +
ggplot2::geom_text(
vjust = -1, aes(x = person_years_label_x, label = person_years_label)
) +
ggplot2::scale_x_continuous(labels = c("", paste0("Q", 1:4))) +
ggplot2::ylim(0.5, 7.5) +
ggplot2::xlab("Calendar quarter") +
ggplot2::ylab(NULL) +
ggplot2::theme_minimal() +
ggplot2::theme(panel.grid.minor = element_blank(),
panel.grid.major.y = element_blank(),
axis.text.x = element_text(hjust = 5),
axis.text.y = element_blank())
person_years_by_quarter_plot <- person_years_by_quarter %>%
ggplot2::ggplot() +
ggplot2::aes(x = quarter_name, y = person_years) +
ggplot2::geom_bar(stat = "identity", width = 1, alpha = 0.5) +
ggplot2::geom_text(aes(label = person_years_label), vjust = -1) +
ggplot2::ylim(0, 2) +
ggplot2::xlab("Calendar quarter") +
ggplot2::ylab(NULL) +
geom_vline(xintercept = 0.5:4.5) +
ggplot2::theme_minimal() +
ggplot2::theme(panel.grid.major.x = element_blank(),
panel.grid.minor.x = element_blank(),
panel.grid.major.y = element_line(color = "darkgray"),
panel.grid.minor.y = element_line(color = "darkgray"),
axis.ticks.y = element_blank(),
axis.text.y = element_blank())
```
```{r individual_person_years_plot, echo=FALSE, message=FALSE, warning=FALSE}
individual_person_years_plot
```
Now, let's look at how those life lines add up to person-years per quarter...
This shows how the person-years changes from quarter to quarter.
```{r person_years_by_quarter_plot, echo=FALSE, message=FALSE, warning=FALSE}
person_years_by_quarter_plot
```
Let's write down what's going on:
$\begin{align}
PY[0,1] &= \textsf{(The total person-years lived that year)} \\
&4 \times 0.25 \textsf{ (4 people alive during Q1 times length of quarter in years)} \\
&6 \times 0.25 \textsf{ (6 people were alive in Q2)} \\
&5 \times 0.25 \textsf{ (5 folks in Q3)} \\
&4 \times 0.25 \textsf{ (4 folks in Q4)} \\
&= `r total_person_years * 4` \times 0.25 \textsf{ (Total number of person-quarters times length of a quarter)} \\
&= `r total_person_years` \text{ (The answer!)}
\end{align}$
Using conventional notation:
$PY[0,1] = \sum_1^4 N_i \times \Delta_i$
* $N_i$: Number of persons alive in the $i$^th^ quarter of the year
* $\Delta_i$: Fraction of a year represented by that quarter (0.25 if the whole quarter is represented)
<br><br>
Or more generally, for $P$ discrete chunks of a period of potentially unequal length:
$PY[0,T] = \sum_1^{P} N_i \times \Delta_i$
<details>
<summary>What would $\Delta_i$ equal if we counted people each day in 2021? **Tap for answer**</summary>
$\frac{1}{365}$
* For 2024, it would be 366 because it's a leap year
* If our rate spanned across a multiple of four years, it would be 365.25 to account for leap and non-leap years
* Why would $\Delta_i$ be tedious to calculate if we did monthly counts?
</details>
<br>
<details>
<summary>What would $N_i$ represent if we counted people each day in a year? **Tap for answer**</summary>
Number of persons alive on the $i$^th^ day of the year
</details>
<br>
<details>
<summary>How could we express $PY[0,1]$ mathematically if we were constantly counting people *ad nauseum* in infinitessimally small units of time of length $dt$? **Tap for answer**</summary>
<br>
$PY[0,1] = \int_0^1 N(t) \cdot dt$
Or for arbitrary period length $T$:
$PY[0,T] = \int_0^T N(t) \cdot dt$
`r emo::ji("nerd")` Hypothetically, the most frequent count cadence possible is each chronon, and theoretically the most frequent count cadence possible is the Planck time, so maybe that integral is in the end a continuous approximation of quantized time. `r emo::ji("nerd")`
</details>
## Principal period rates in demography
* All of these rates are for an entire population
* For each rate, think about this mangled quote from PHG pg. 7 ¶ 5:
> As is especially clear from our definition of the crude rate of in-migration, the connection between exposure and event is not always precise in demography
Crude birth rate:
$$\begin{align}
CBR[0,T] &= \frac{\textsf{Number of births between times } 0 \textsf{ and } T}
{\textsf{Person-years lived between times } 0 \textsf{ and } T} \\
&= \frac{B[0,T]}{PY[0,T]}
\end{align}$$
Crude death rate:
$$\begin{align}
CDR[0,T] &= \frac{\textsf{Number of deaths between times } 0 \textsf{ and } T}
{\textsf{Person-years lived between times } 0 \textsf{ and } T} \\
&= \frac{D[0,T]}{PY[0,T]}
\end{align}$$
Crude rate of in-migration:
$$\begin{align}
CRIM[0,T] &= \frac{\textsf{Number of in-migrations between times } 0 \textsf{ and } T}
{\textsf{Person-years lived between times } 0 \textsf{ and } T} \\
&= \frac{I[0,T]}{PY[0,T]}
\end{align}$$
Crude rate of out-migration:
$$\begin{align}
CROM[0,T] &= \frac{\textsf{Number of out-migrations between times } 0 \textsf{ and } T}
{\textsf{Person-years lived between times } 0 \textsf{ and } T} \\
&= \frac{O[0,T]}{PY[0,T]}
\end{align}$$
![CROM!](./images/crom-approves.jpg)
## Growth rates in demography
Measuring population growth has many uses in traditional demography, such as:
**Assessing and mitigating risk of Malthusian traps in resource allocation due to diminishing marginal returns**
![From CORE econ's *The Economy* textbook, Figure 2.15: https://www.core-econ.org/the-economy/book/text/02.html#malthusian-economics-the-effect-of-technological-improvement](images/figure-02-15-core-econ.svg){width=100%}
**Assessing and mitigating risks that slow growth leads to population aging**
* What impact does this have on the Social Security system?
* What impact does this have on age patterns of socio-political power?
![From U.S. Census Bureau: https://www.census.gov/library/stories/2021/12/us-population-grew-in-2021-slowest-rate-since-founding-of-the-nation.html](images/us-population-grew-in-2021-slowest-rate-since-founding-of-the-nation-figure-2.webp){width=100%}
![From U.S. Census Bureau: https://www.census.gov/library/visualizations/2018/comm/century-of-change.html](images/pop-projections-3-800.webp){width=100%}
Outside of human demography, tracking how human consumption impacts food stock growth is important and sometimes depressing:
![By Food and Agriculture Organization of the United Nations (FAO) - http://www.fao.org/3/ca0191en/ca0191en.pdf, CC BY-SA 3.0 igo, https://commons.wikimedia.org/w/index.php?curid=77504632](images/fish-stocks.svg){width=100%}
:::{.rmdtip}
**DEMOGRAPHY & DATA SCIENCE**
#### Customer demand vs. workforce supply: Challenges to company scale {.unnumbered}
If high attrition causes employee headcount to grow too slowly relative to demand
* Not enough workers to meet customer demand
* Increased costs to backfill workers who leave
* If your hiring rate is also very high, increased risk of labor market saturation
> Even before the pandemic, previously unreported data shows, Amazon lost about 3 percent of its hourly associates each week, meaning the turnover among its work force was roughly 150 percent a year. That rate, almost double that of the retail and logistics industries, has made some executives worry about running out of workers across America.
>
> -- Reporting by Jodie Kantor, Karen Weise, and Grace Ashford in the New York Times: https://www.nytimes.com/interactive/2021/06/15/us/amazon-workers.html
:::
### Crude growth rate
The crude growth rate combines the [Principal period rates in demography] into an expression of how a population grows between two time points
Recall the balancing equation:
$$N(T) = N[0] + B[0,T] - D[0,T] + I[0,T] - O[0,T]$$
Using some high school algebra, replace the "?" in the equation below:
<details>
<summary>$$
\frac{\textsf{?}}{PY[0,T]} =
\frac{B[0,T]}{PY[0,T]}
- \frac{D[0,T]}{PY[0,T]}
+ \frac{I[0,T]}{PY[0,T]}
- \frac{O[0,T]}{PY[0,T]}
$$ **Tap for answer**</summary>
<br>
$N(T) - N(0)$ (The period change in population count)
Step 1: Subtract $N(0)$ from both sides
Step 2: Divide both sides by person-years $PY[0,T]$
<br>
</details>
<details>
<summary>Putting it all together and substituting in the principal period rates:
$$CGR[0,T] = \frac{N(T) - N(0)}{PY[0,T]} = \textsf{?}$$ **Tap for answer**</summary>
$$CBR[0,T] - CDR[0,T] + CRIM[0,T] - CROM[0,T]$$
</details>
<details>
<summary>Arranging by rates related to enters vs. exits **Tap for answer**</summary>
$$\begin{align}
CGR[0,T] &= \\
&CBR[0,T] - CDR[0,T] \textsf{ (Crude rate of natural increase } CRNI[0,T] \textsf{)}\\
&+ CRIM[0,T] - CROM[0,T] \textsf{ (Crude rate of net migration } CRNM[0,T] \textsf{)}
\end{align}$$
</details>
### Instantaneous growth rate
CGR measures growth between two time points $0$ and $T$, but what about the pace of population growth at a specific point in time $0 < t < T$?
* Define period length $\Delta t$
* Population change $N(t + \Delta t) - N(t) = \Delta N(t)$ (Just a generalization of $N(1) - N(0)$)
* Person-years lived now $N(t)\Delta t$ (Note similarity to $PY[0,T] = \sum_1^{P} N_i \times \Delta_i$. Here, $P = 1$ since we are concerned with only one small interval of time for the whole period.)
* So crude growth rate now $r(t) = \frac{\Delta N(t)}{N(t) \Delta t}$
:::{.rmdimportant}
**KEY CONCEPT**
Instantaneous growth rate is essentially a crude growth rate over tiny period of length $\Delta t \rightarrow 0$
Mathematically^[I'll spare you the derivation from PHG pg. 9, but if you're interested in it and confused about it, talk to me during office hours.]:
\begin{equation}
r(t) = \lim_{\Delta t \to 0} \frac{\Delta N(t)}{N(t) \Delta t}
=\frac{d \text{ln}\left[N(t)\right]}{dt}
(\#eq:instantaneousgrowthrate)
\end{equation}