Wednesday, May 17, 2006

The population growth circus

I think it all started with the title of the unfortunate link title of the NSCB press release: "Philippine population growth slows to 1.95 percent by 2010." (This is already incorrect, but the newshounds started tracking.) The content of the press release itself clearly states that the 1.95% figure, covering the period 2005-2010, is a projection.

Somehow reporters got stuck with the link title and dropped the "estimate" part.

Reality check: the only way to check for sure whether population growth really slow down is to conduct a census. The last one was in 2000. The next one is scheduled for 2010. There is no way to check the actual population growth per year in between. (Censuses are expensive.)

The confusion has gone way out of hand, as discussed in the NSCB clarification. Even NEDA chief Romulo Neri got carried away:
Romulo Neri, director general of the National Economic and Development Authority (NEDA), noted that the latest population growth rate was nearing the government’s medium-term target of 1.94 percent. Neri said slowing down population growth to 1.94 percent a year was necessary to enable the Philippine economy to feed and sustain its entire population.
Unless of course he was misquoted, which is perfectly possible.

Solita Monsod finds the projection unbelievable, and so does Dean Jorge Bocobo. Essentially both are arguing that the decline simply does not square with historical data. (I should hat-tip DJB for getting me to post on this topic.)

However, the population projection method is not based on fitting to past data. It uses projections of fertility rate and mortality, using baseline data from the 2000 Census, combined with certain scenarios. For the mortality rate, the projection applies life expectancy, with an assumed upward increment over time (about 2 years for every quinquennium). For the fertility rate, three assumptions are made, regarding the year in which net replacement fertility (approximately zero population growth) is reached: 2030 for the low assumption, 2040 for the medium assumption, and 2050 for the high assumption. The estimates cited by the NSCB pertain to the medium assumption.

I am not a demography expert. However I would agree with DJB and Mareng Winnie that some consistency be observed with experience. To my amateur eyes, I would think that pushing the target dates of net replacement fertility backward would maintain the official method, while satisfying critics. Perhaps by ten years? DJB has a graph showing that using the "high" as the working assumption leads to a better fit with historical data.

Lessons learned from this brouhaha:

1. Journalists are seldom to be trusted for accuracy in reporting crucial technical details. If possible one must always go back to the source document (often a technical report, or a journal article).

2. A fantastic amount of saliva and ink can be spilled, largely on inane discussions by politicians and other "concerned citizens", about population programs, birth control, public investment priorities, the Roman Catholic church, and so forth, on the basis of what is essentially an urban legend.

3. Be careful about naming your hyperlinks!!


Rizalist said...

My earlier comment did not appear on your blog when I put it here last week but...

Congratulations on this great post. I realized because of it that my first post on the subject may have misled some readers by not emphasizing the actual methodology used by NSCB. Which you've explained better than I did. Thanks for the heads up Econblogger!

Rizalist said...

It's really too bad the media dumbs down a lot of this stuff and doesn't get it out to their readers.

I actually appreciate Winnie's forays into this area and have a lot o respect for her work.

It is the limitations of the Main STream Media that actually forces writers like her to simply avoid certaint topics.

I'm sure she has some understanding of differential calculus (it is centuries old after all!)