## Population Projections

This computing log illustrates the use of the cohort component method in population projections.

### Reading the Data

We will use the data from Sweden used in Box 6.1 (page 125 in the textbook), available as file
`sweden93.dat`

in the datasets section to save you some typing. We read the population counts,
the life table, and the fertility rates. The last age group is 85+ and `L`

for that age is
actually time lived after age 85. We also express person-years per birth, dividing by
*l _{0}*.

. infile age p L f using /// > http://data.princeton.edu/eco572/datasets/sweden93.dat (18 observations read) . mata ------------------------------------------------- mata (type end to exit) -------------- : p93 = st_data(., "p") : L = st_data(., "L")/100000 : f = st_data(., "f") : sum(p93) 4397428 : end ----------------------------------------------------------------------------------------

> sw <- read.table("http://data.princeton.edu/eco572/datasets/sweden93.dat", header=FALSE) > names(sw) <- c("age", "p93", "L", "f") > sw <- mutate(sw, L = L/100000) > summarize(sw, sum(p93)) sum(p93) 1 4397428

### The Leslie Matrix

We will write a function to compute a Leslie matrix given two vectors representing person-years
lived * _{5}L_{x}* (with the convention that the radix is one and the last value
is

*T*) an the maternity rates

_{x}*. (To simplify cutting and pasting I provide the code rather than a log.)*

_{5}m_{x}real matrix Leslie(real vector L, real vector m) { n = length(L) M = J(n,n,0) // lower diagonal has survivorship ratios for (i=1; i < n; i++) { M[i+1,i] = L[i+1]/L[i] } M[n,n-1] = M[n,n] = L[n]/(L[n-1]+L[n]) // first row has net maternity contributions for(i=1; i < n; i++) { if(m[i]==0 & m[i+1]==0) continue M[1,i] = L[1]*(m[i] + m[i+1]*L[i+1]/L[i])/2 } if (m[n] > 0) M[1,n] = L[1]*m[n] return(M) }

Leslie <- function(L, m) { n = length(L) M = matrix(0, n, n) # lower diagonal has survivorship ratios for (i in 1:(n-1)) { M[i+1,i] <- L[i+1]/L[i] } M[n,n-1] <- M[n,n] <- L[n]/(L[n-1] + L[n]) # first row has net maternity contributions for(i in 1:(n-1)) { if(m[i] != 0 | m[i+1] != 0) { M[1,i] <- L[1] * (m[i] + m[i+1] * L[i+1]/L[i])/2 } } if (m[n] > 0) M[1,n] <- L[1] * m[n] M }

The function computes the survivorship ratios and stores them in
the lower diagonal. For example `M[2,1] = L[2]/L[1]`

is the probability of surviving from age 0-4 to 5-9, or
* _{5}L_{5}/_{5}L_{0}* in
standard demographic notation.

The only tricky bit is the last (open) age interval, where we
combine the last two groups and project then using
`L[n]/(L[n-1] + L[n])`

.
In Sweden we combine ages 80-84 and 85+ and use
*T _{85}/T_{80} =
T_{85}/(_{5}L_{80}+T_{80})*
as the survival ratio. (As noted in class, if we had

*T*we would use a slightly different procedure. The textbook describes that procedure on page 121, but uses the combined projection in Box 6.1.)

_{90}The rest of the calculation computes the average fertility rate
for each age group and then survives the resulting births to age
0-4. For example women 15-19 are exposed to the rates at 15-19
and 20-24, with the latter discounted by the probability of
surviving to 20-24, so we average
`m[4]`

and `m[5]*L[5]/L[4]`

.

The rate would be multiplied by 5, the width of the period,
and by the probability of surviving from birth to age 0-4, which
is `L[1]/5`

. The 5's cancel, so we don't include them.
(As noted in class there are two ways of computing births;
here we focus on the women and average the rates, an approach
best suited for computing the Leslie matrix. The textbook also
describes focusing on the rates and averaging the numbers of women
exposed to each rate.)

### Projections for 5 and 10 years

For Sweden we have fertility rates, so we divide by 2.05 to obtain
*maternity* rates (female births) before calling our function.
Once we have a Leslie matrix, projection is easy

. mata ------------------------------------------------- mata (type end to exit) -------------- : M = Leslie(L, f/2.05) : p98 = M * p93 : p03 = M * p98 : round( (p93, p98, p03) ) 1 2 3 +----------------------------+ 1 | 293395 293574 280121 | 2 | 248369 293189 293368 | 3 | 240012 248251 293049 | 4 | 261346 239833 248066 | 5 | 285209 261015 239529 | 6 | 314388 284787 260629 | 7 | 281290 313782 284238 | 8 | 286923 280463 312859 | 9 | 304108 285576 279147 | 10 | 324946 301731 283344 | 11 | 247613 320974 298043 | 12 | 211351 243039 315045 | 13 | 215140 205109 235861 | 14 | 221764 204944 195388 | 15 | 223506 204793 189260 | 16 | 183654 194419 178141 | 17 | 141990 142324 150666 | 18 | 112424 131768 141960 | +----------------------------+ : sum(p98), sum(p03) 1 2 +---------------------------+ 1 | 4449569.73 4478712.28 | +---------------------------+ : end ----------------------------------------------------------------------------------------

> M <- Leslie(sw$L, sw$f/2.05) > sw <- mutate(sw, + p98 = M %*% p93, + p03 = M %*% p98) > round(select(sw, age, p93, p98, p03), 0) age p93 p98 p03 1 0 293395 293574 280121 2 5 248369 293189 293368 3 10 240012 248251 293049 4 15 261346 239833 248066 5 20 285209 261015 239529 6 25 314388 284787 260629 7 30 281290 313782 284238 8 35 286923 280463 312859 9 40 304108 285576 279147 10 45 324946 301731 283344 11 50 247613 320974 298043 12 55 211351 243039 315045 13 60 215140 205109 235861 14 65 221764 204944 195388 15 70 223506 204793 189260 16 75 183654 194419 178141 17 80 141990 142324 150666 18 85 112424 131768 141960 > select(sw, p93, p98, p03) %>% colSums() p93 p98 p03 4397428 4449570 4478712

These results agree exactly with Box 6.1 (part 2) in the text.

### The Stable Equivalent

While we have the Leslie matrix handy we can compute the intrinsic rate of growth and the stable age distribution by simply obtaining the first eigenvalue and eigenvector of the projection matrix.

This is easy to do in Mata, as long as we define the output vector and matrix before. Here I use empty matrices.

. mata ------------------------------------------------- mata (type end to exit) -------------- : values = J(0,0,.) : vectors = J(0,0,.) : eigensystem(M,vectors,values) : values[1] 1.00111254 : log(values[1])/5 .000222384 : stable = Re(vectors[,1]/sum(vectors[,1])) : stable 1 +---------------+ 1 | .0619116758 | 2 | .061799489 | 3 | .0617013824 | 4 | .0615869209 | 5 | .0614405485 | 6 | .0612814306 | 7 | .0610952702 | 8 | .0608479528 | 9 | .0604950315 | 10 | .0599554176 | 11 | .0591567526 | 12 | .0579994633 | 13 | .0562239225 | 14 | .0534997793 | 15 | .0493506336 | 16 | .042880453 | 17 | .0331935312 | 18 | .0355803452 | +---------------+ : end ----------------------------------------------------------------------------------------

> e <- eigen(M) > e$values[1] [1] 1.001113+0i > log(e$values[1])/5 [1] 0.0002223831+0i > arv <- abs(Re(e$vectors[,1])) > stable <- arv/sum(arv) > stable [1] 0.06191167 0.06179949 0.06170138 0.06158692 0.06144055 0.06128143 [7] 0.06109527 0.06084795 0.06049503 0.05995542 0.05915675 0.05799946 [13] 0.05622392 0.05349978 0.04935063 0.04288045 0.03319353 0.03558035

The first eigenvalue is 1.0011, so if the 1993 fertility and mortality rates
were to stay constant the population would eventually grow 0.11 percent
every 5 years, or 0.022 percent per year. The first eigenvector, divided by
its sum, gives us the proportionate age distribution.
In general the eigenvalues and eigen vectors can be complex numbers and both
Stata and R store them as such, although for a Leslie matrix they are always
real. The function `Re()`

takes the real part of a complex number.

We return to stable populations in a separate handout, but there is a lot you could do with the Swedish data. For one thing you might try projecting the population for 100 years of so (20 periods) to verify that it becomes stable. You may also try plotting the current and stable equivalent age distributions to see what that says about population momentum.