 Global Data Reliability, 1990 {Robinson Projection} |
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Temporal Reliability Maps
The population counts are determined by the interpolation or extrapolation of data for using one or two reference years. First, a rate of growth between the reference years is determined as follows:
where r is the average rate of growth, P1 and P2 are the population totals for the first and second reference years, and t is the number of years between the two census enumerations. This rate is then applied to the census figures to interpolate or extrapolate population totals in the base years of 1990 and 1995. For example, the 1995 estimate is calculated: P1995 = P1 ert This method of interpolation or extrapolation assumes a constant rate of growth for the years between the intervals, an assumption which is not true especially under conditions under rapid population growth or decline. In general, for which actual population counts or an official estimate were available. Grids estimates tend to be moreare more accurate if reference years match closely the gridding datebase years. For the purpose of mapping this source of errors in the input data, an index of variable called "reliability" has been calculated. Reliability is inversely proportional with the distances from reference years. It isThe index scaled scales to cover the range of values between 0 and 2. The value maximizes at 2 if one of the reference years coincide with the gridding yearbase year. If the reference year was far in the past (or future), the confidence reliability of the actual valueestimate being the one obtained through interpolation or extrapolation is low because of the assumptions built into estimating uniform change. The index of reliability is calculated, separately for each base year, as follows: Where Rb is reliability in year time, b is a base year, Y1 is the year of the first estimate and Y2 is the year of the second estimate. Most countries had two national estimates of population counts. For countries where only one official national estimate was available, UN estimates were used as one of the reference years for interpolation or extrapolation. The UN has estimates for all countries for both base years. The index of reliability is only concerned with official national estimates, thus those with only one data point have lower scores. We were able to obtain two base year estimates for a large majority of countries. Thirty-eight countries had only one population estimate. This includes newly formed states (e.g., Croatia, Palestinian National Authority) as well as countries for either economic or political reasons have not conducted a census or released census results since 1990 (e.g., Afghanistan, Albania). For countries with one or no population estimates (as in the case of most of the Pacific and Caribbean Islands), the UN estimate(s) are used for either one or both of the reference years. Only a few larger countries had no population estimates (e.g., Bosnia Herzegovina, Kuwait, Singapore) that were available to us. Details on the sources for the population data are listed in the country-specific documentation. For a small share of the countries (14 of the total, or roughly 6%), we had census estimates for each of these years. Of course, this is not the only measure of reliability. In addition to spatial resolution, data for the reference year may be an official estimate of uncertain accuracy (e.g., in Angola). Also, in some instances, even official census data for a given year may be of uncertain reliability—e.g., Nigeria or Gambia. In some cases, the uncertainty involved in the interpolation or extrapolation of population data values may often be lower than the uncertainty in the original census figures or estimates. Users concerned about these issues of uncertainty would be well served by using the UN-adjusted data.
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