In this paper, a methodology to estimate global migrant transition flow tables is illustrated in two parts. First, a methodology to derive flows from sequential stock tables was out- lined. Second, the methodology was applied to recently released World Bank migration stock tables to estimate a set of four decadal global flow tables. The results of the ap- plied methodology were discussed with reference to comparable estimates of global net migration flows of the United Nations and previous models for international migration flows.
The methodology outlined adds to the limited exisitng literature on linking migration flows to stocks. Rogers and von Rabenau (1971), Rogers and Raymer (2005) and Rogers and Liu (2005) focused on US Census place of birth data to estimate inter-state migration transitions. The first of these papers relies on a simplistic principal assumption of equal growth of stock totals in all regions. As a result, the application of the method outlined to the World Bank data produced many strange results, including negative migration flows. The second paper relies on aggregated stock populations by their place of birth, place of residence at timet−1, and place of residence at timet. This added dimension of required data negates its application to estimating international migration flows. This point is also valid for the third paper, which relies on information of migrant stocks disaggregated by their place of residence at timet−1, place of residence at timet, and age group.
The method for estimating migrant transition flows from stocks demonstrated in this paper constrains results to match those of known sequential stock tables. This approach is beneficial for a number of reasons. First, comparable migration stock data are easier to measure and collect at a global level than flow data. Difficulties in producing comparable international migration flow data have recently lead to a number of competing estimation efforts, all applied to European data, see for example Abel (2010); Beer et al. (2010); Raymer et al. (2012). However, these methodologies rely on a reasonable percentage of double counted flows, i.e., reported values of movements from both the sending and re- ceiving counties. The application of these estimation methods to obtain global migration flow data is hindered by the availability of reported migration flows, which from non- European countries remains scarce. Second, as discovered in the results section of this paper, estimating flows from stocks can provide a good check on the stock data itself. The flow estimation method is partly reliant on the differential change in the stock data. When this change is large, large flows are estimated. Where these flow estimates are unexpected, peculiarities in the data can be quickly identified. Third, estimating flows required to match stock totals results in migrant transitions rather than movement events. Transition measures can be of particular use in global population projection models, such as those outlined by Cohen et al. (2008). Projection models tend to require migration tran- sitions that match the age groups at which the future population is predicted. For example,
the results presented in this paper could potentially be incorporated into projection models that age populations at 10-year intervals. If denser intervals are desired, such as five-year or single-year age intervals, new estimates would be required. These could be derived in one of two ways. Stock tables in non-census years could be estimated to create a larger set of sequential stock tables (within the same time frame), allowing flows between closer time periods to be obtained. Alternatively, one could adapt a method such as that outlined by Courgeau (1973) to convert 10-year estimates to shorter periods. If such estimates were derived, they could potentially be compared with flow data from countries that have a Census question on the place of residence one or five years prior to their Census night, forming another conceivable validation exercise.
The estimated flow tables are a first-of-a-kind set of comparable global origin desti- nation flow data. Estimates represent the minimum number of migration transition flows required to meet the foreign born stocks. As discussed in the introduction, bilateral flow tables have a number of advantages over currently available international migration data. Stock data are collected in the country of residence via census questions or details col- lected from population registers which capture a single transition between birthplace and country of current residence. However, the methodology outlined allows flows to be esti- mated on three dimensions; origin, destination and place of birth. Consequently, multiple typologies of movements such as primary migration, return migration or onward migra- tion flows of non-natives to a third country are estimated. Results from both validation exercises appeared promising. The validation exercises allowed, as did the brief overview of the main results, the identification of unexpected flows. For most cases, further inves- tigation of these unexpected results was driven by large changes in the input stock data. In other cases, the methodology could be further expanded in a number of different ways to address the causes of these unexpected flows.
Estimated flows were minimised by making an assumption on the number of stayers. Empirical evidence, where available, could be utilised to reduce the diagonal element in each birthplace specific table, producing more migration flows to and from a selected country. Discrepancies in the total number of two sequential stock tables were first altered to account for natural population change. Remaining differences were assumed to be due to moves to or from external countries not studied. In reality, in the application of the methodology the majority of these differences were most likely due to measurement issues in the stock data. The methodology could be expanded to incorporate measures of under or over counting of stocks, where known, to reduce the amount of external moves that are proportionally allocated. The World Bank stock data were taken to represent the foreign born stock totals at the start of each decade. In reality, data was actually collected over a number of years around each census period. Corrections for stock data that allow estimates of foreign born stocks at one point of time for all countries might further reduce some of the unexpected flow estimates. Changes in stocks from natural population change
were controlled for using rather crude methods. More detailed data are likely to exist in some counties on the mortality by place of birth of foreign born migrants that may negate the need to distribute the total number of deaths proportionally. In addition, the estimation of the place of residence of newborns could be further developed to allow for moves outside their country of birth in these early years. Finally, the log-linear models with an offset have been used to estimate missing internal migration flow data. Alternative auxiliary data, to the distance measure used in this paper, could be utilised within the methodology outlined.
In conclusion, comparable international migration flow data are needed by researchers to better understand people’s movements and identify patterns. Policy makers can also use comparable international migration flow data to help forecast populations better, where migration can often play an important role. The methodology outlined in this paper pro- vides a relatively simple yet powerful technique to estimate global migration flow tables, exploiting newly available global stock data.
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Acknowledgements
I would like to thank James Raymer, Nikola Sander, and Jack DeWaard for their com- ments on an earlier version of this paper and Joel Cohen for his recommendations con- cerning the validation exercises. I would also like to thank the reviewers and the Associate Editor for further suggestions on improving the paper.
Corrections:
On June 3, 2013 several typing mistakes were corrected on pages 521, 522, and on page 524.