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130
The terms population statistics1 or demographic statistics1 (102-2), when used in the singular, denote methods of quantitative analysis of population data, or, more generally, the art of collecting and presenting statistical information about the population. When used in the plural (cf. 102,2) they refer to numerical data2 about populations, which are based on observations3. After such observations have been collected4, edited5 or scrutinized5 to eliminate obvious inconsistencies, they are tabulated6 by grouping7 or classifying7 (cf. 221-5) them into different groups8 or classes9 (cf. 116-2*). The processes from editing to tabulation are sometimes referred to as the collation9 of data.
- 1. statistics n. — statistical adj. — statistician n., a specialist in statistics.
- 4. collect v. — collection n,
- 5. edit v. — editing n. scrutinize v. — scrutiny n.
- 6. tabulate v. — tabulation n.
- 7. classify v. — classification n. 9. collation n. — collate v.
131
The basic data1, raw data1, primary data1, or crude data1 consist of a series2 of numbers3 or figures3 (cf. 155-2) which are put together in the form of a statistical table4. In such a table the data are generally classified with respect to certain variables5 or variates5 such as age, number of children, etc., or with respect to certain qualities6, characteristics6 or attributes6 such as sex. Where data are classified with respect to several variables or attributes simultaneously the tables are called cross-tabulations7. Summary tables8 give information in rather less detail than do individual tables9.
132
For purposes of analysis1 it is usually necessary to obtain more refined figures2 from the raw data (131-1). The first stage in the analysis of data frequently consists of the computation3 of ratios, proportions, percentages, or rates (cf. paragraph 133). The last three are particular types of ratio.
- 1. analysis n. — analytical adj. — analyse v.
- 3. computation n. — compute v, — computer or computer n., one who is engaged in computation. The term "computer" has recently also been used for computing machines.
133
A ratio1 is a quotient which indicates the relation in size of one number to another. A proportion2 is a ratio which indicates the relation in magnitude of a part to the whole. A percentage3 is a proportion in a hundred. A rate4 was initially a special type of ratio used to indicate the relative frequency5 (cf. 144-3) of the occurrence of a particular event within a population or sub-population. The term has, however, acquired a steadily wider meaning and is often used as a synonym for ratio. It is nowadays used in many different senses but the concept of ratio is common to them all.
- 2. proportion n. — proportional adj.
- 4. Rates are generally given per thousand, and where the term "rate" is used without qualification this is understood. Some rates, however, are given per ten thousand, per hundred thousand or per million, e.g. cause mortality rates (cf. 421-7). On other occasions rates may be given per head or per person. The word "rate" is sometimes omitted, thus one may find the expression "a mortality of ten per thousand", but this is not recommended.
134
The relative frequency (133-5) of an event is often regarded as the probability1 of occurrence of that event. If this is done, all the individuals who appear in the denominator must have been exposed to risk3 in some way, i. e., there must have been a chance2 or risk2 that the event in question could happen to them. The use of the term "risk" or "chance" does not imply that the event in question is in any way unwanted — thus the term "risk of marriage" is used. The population is often divided into different sub-groups, in which the risk of the event in question is less variable between individuals than in the population as a whole; the subgroup is more homogeneous4 with respect to the risk than the relatively heterogeneous5 whole population. Rates calculated for such sub-groups are called specific rates6, as opposed to general rates7 or crude rates (135-8) which apply to the population as a whole.
- 1. probability n. — probable adj.
- 4. homogeneous adj. — homogeneity n.
- 5. heterogeneous adj. — heterogeneity n.
135
Data are called provisional1 if they are based on incomplete or insufficiently controlled observations. They are replaced by final2 data when the observations are complete. Rates based on such data are called provisional rates3 and final rates4 respectively. Where unexpected information becomes available after figures have already been published, revised rates5 may be issued. The expression corrected rate6 usually implies that defective data or inappropriate methods have yielded results which are either misleading or of limited value for the purpose in hand and that an effort has been made to correct this, e. g. correction for under-enumeration, correction for migration, correction for seasonal movement. Standardized rates7 or adjusted rates7 are designed to make it possible to compare different populations with respect to a variable, e. g. fertility or mortality, where the influence of another variable e. g. age, is held constant. The term corrected rate7 has been used by some demographers as a synonym for standardized rate. Unstandardized rates are called crude rates8. Although they may be used to measure actual trends, false inferences may result from their uncritical use when populations with different structures (144-4) are compared.
(It should be emphasized that there is no uniformly accepted terminology for the subjects mentioned in this paragraph and that the terms are often used rather loosely.)
136
In its most general sense, the term index1 (pl. indexes or indices) or the term indicator1, is employed for any number measuring a given quantity; but in a more restricted sense an index2 is a ratio showing the value of a given quantity relatively to a base3, which is conventionally taken as 100.
137
Demographic indices (136. 1) will in most cases relate to a particular period of observation1; this is true in particular of most rates (133-4). An annual rate2 will relate to a period of twelve months. Where observations are collected for a number of years and then averaged, the term mean annual rate3 is often used for the result. Where rates are calculated for periods different from a year they are often converted to an annual basis4 through multiplication by an appropriate factor. Instantaneous rates5 are sometimes computed; they relate to an infinitesimal period of time, cf. for instance the instantaneous death rate (432-4) or the instantaneous rate of population growth (701-1).
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