The Demopædia Encyclopedia on Population is under heavy modernization and maintenance. Outputs could look bizarre, sorry for the temporary inconvenience
Multilingual Demographic Dictionary, second unified edition, English volume
15
Disclaimer : The sponsors of Demopaedia do not necessarily agree with all the definitions contained in this version of the Dictionary. The harmonization of all the second editions of the Multilingual Demographic Dictionary is an ongoing process. Please consult the discussion area of this page for further comments. |
Go to: Introduction to Demopædia | Instructions on use | Downloads |
150
When values of a demographic variable are available over time, a demographic time series ^{1} is obtained. It is sometimes possible to decompose a time series into a trend ^{2} around which there are fluctuations ^{3}, variations ^{3}, or deviations ^{3} (141-2). Where such fluctuations tend to recur after certain periods, usually several years, they are called cyclical fluctuations ^{4} or, more generally, period fluctuations ^{4}. In demography the most common period for compiling data is a year, and the fluctuations in sub-periods of a year are called seasonal fluctuations ^{5}. The fluctuations that remain after trend, cyclical, and seasonal fluctuations have been eliminated are called irregular fluctuations ^{6}. They may be due to exceptional factors such as wartime mobilization, or sometimes they are chance fluctuations ^{7} or random fluctuations ^{7}.
- 3. In a general sense the term variation may be used to describe change in any value or set of values for a variable.
- 4. Periodic, adj. - period, n. - periodicity, n. cyclical, adj. - cycle, n.
- 7. Random, adj.: due to chance (cf. 161-1).
151
It is occasionally desirable to replace a series of figures by another series that shows greater regularity. This process is known as graduation ^{1} or smoothing ^{1}, and it generally consists of passing a smooth curve through a number of points in the time series or other series, such as the number of persons distributed by reported age. If a free-hand curve is drawn the process is called graphic graduation ^{2}. When analytical mathematical methods are used, this is called curve fitting ^{3}. A mathematical curve is fitted to the data, possibly by the method of least squares ^{4}, which minimizes the sum of the squares of the differences between the original and the graduated series. Other methods include moving averages ^{5} or involve the use of the calculus of finite differences ^{6}. Some of these procedures may be used for interpolation ^{7}, the estimation of values of the series at points intermediate between given values, or for extrapolation ^{8}, the estimation of values outside of the range for which it was given.
- 1. Graduation, n. - graduate, v. - graduated, adj. Smoothing, n. - smooth, v. - smoothed, adj.
- 7. Interpolation, n. - interpolate, v. - interpolated, adj.
- 8. Extrapolation, n. - extrapolate, v. - extrapolated, adj.
152
It is often necessary to graduate distributions to correct the tendency of people to give their replies in round numbers ^{1}. Heaping ^{2} or digit preference ^{2} can be found in various duration distributions (e.g., distribution of marriage duration, breastfeeding duration or birth intervals), but is particularly frequent in age distributions and reflects a tendency for people to state their ages in numbers ending with 0, 5, or other preferred digits, but . Age heaping ^{3} is sometimes measured with indices of age preference ^{4}. Age data must often be corrected for other forms of age misreporting ^{5} or age reporting bias ^{5}
153
The numerical values of demographic functions are generally listed in tables ^{1}, such as life tables (431-1), fertility tables (634-1), or nuptiality tables (522-1). A distinction is usually made between calendar-year tables ^{2} or period tables ^{2} which are based upon observations collected during a limited period of time, and cohort tables ^{3} or generation tables ^{3} which deal with the experience of a cohort throughout its lifetime. A multiple decrement table ^{4} illustrates the simultaneous effects of several non-renewable events, such as the effects of first marriage and death on the single population. The most used are double decrement tables ^{4}. Forecast tables ^{5★} provide numerical values of demographic functions, like survival functions (431-6) for example, which can be used directly for population forecast (cf. 720-2). When a population is classified in two or more categories according to age, like economic status (women in the labor force or out of the labor force, for example), marital statuses, regions etc. and when continuous flows between categories are possible over time even if the individual state can usually be measured only at discrete times (waves of a longitudinal study, queries to population registers etc.), increment-decrement methods ^{6★} or multi-state methods ^{6★} are more appropriate.
154
Where insufficient data exist to establish the value of a given variable accurately, attempts may be made to estimate ^{1} this value. The process is called estimation ^{2} and the resulting value an estimate ^{3}. Where data are practically non-existent a conjecture ^{4} may sometimes be made to establish the variable’s order of magnitude ^{5} .
155
Methods of graphic representation ^{1} or diagrammatic representation ^{1} may be used to illustrate the data. The data are represented in a figure ^{2}, graph ^{2}, statistical chart ^{3} or map ^{3}. A schematic representation of the relationships between variables is often called a diagram ^{4}, for example the Lexis Diagram (cf. 437). A graph in which one co-ordinate axis is graduated logarithmically and the other arithmetically is called a semi-logarithmic graph ^{5}, though such graphs are often inaccurately referred to as logarithmic graphs ^{5}. A true logarithmic graph ^{6} has both axes graduated logarithmically and is sometimes referred to as a double logarithmic graph ^{6}. A frequency distribution may be represented graphically by frequency polygons ^{7} obtained by joining points representing class frequencies with straight lines, by a histogram ^{8}, where class frequencies are represented by the area of a rectangle with the class interval as its base, by bar charts ^{9}, in which the class frequencies are proportionate to the length of a bar or by an ogive ^{10} representing the cumulative frequency distribution.
* * *
Go to: Introduction to Demopædia | Instructions on use | Downloads |