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To arrive at the 1980 population projection use percentage changes for 19401950, 19501960, and 19601970 and proceed in same manner as above. CohortSurvival Method In making population projections by age, for any area, from one decennial census to another, the simplest and best formula for all age groups, except those born during the projection decade, is P7 = (P6 X PG)/PJ x x10 x x10 7 where P is the population of age group x in 1970 P is the population of age group x in 1960 x 6 P is the population of age group x10 in 1960 x10 P'* is the population of age group x10 in 1950. x10 The assumptions underlying the use of this short projection formula are: 1) there are no changes in the relevant definitions of the population or in the boundaries of the area whose population is being projected. 2) Age specific rates of mortality and migration do not change between the past, decade and the next decade. 3) The effect of errors of enumeration for the past two censuses will be the same in taking the next census. Although there may be room for argument about the nature of these assumptions, they are. essentially either the same or quite similar to the assumptions which must be made implicitly in other kinds of projections by age. UNDER THE ASSUMPTIONS STATED ABOVE, THE FORMULA IS EXACT. It does not require explicit separate adjustments for mortality and migration. Better still, the formula has built into it the LOCAL combined mortality and migration rates of the base decade 19501960. Hence, there is no need to adjust for differences between mortality of the local area and that of the state or nation as is necessary in the use of U. S. Census survival rates in estimating net migration in local areas. For the same reason, the basic formula may be used with race and sex classifications without adjustments of any kind. The above formula is more obvious when it is stated in such a manner that, for example, the ratio of white males ages 3034 in 1970 is to that, same group, ages 2024 in i960, as white males ages 3034 in 1960 are to the same group, ages 2024 in 1950. Obviously, the ratios are functions of the effects of migration and mortality operating during the two decades. They may be equal to, more than, or less than unity, depending on the relative amount of net migration and mortality during the decade. in
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Title  Page 31 
Full Text  To arrive at the 1980 population projection use percentage changes for 19401950, 19501960, and 19601970 and proceed in same manner as above. CohortSurvival Method In making population projections by age, for any area, from one decennial census to another, the simplest and best formula for all age groups, except those born during the projection decade, is P7 = (P6 X PG)/PJ x x10 x x10 7 where P is the population of age group x in 1970 P is the population of age group x in 1960 x 6 P is the population of age group x10 in 1960 x10 P'* is the population of age group x10 in 1950. x10 The assumptions underlying the use of this short projection formula are: 1) there are no changes in the relevant definitions of the population or in the boundaries of the area whose population is being projected. 2) Age specific rates of mortality and migration do not change between the past, decade and the next decade. 3) The effect of errors of enumeration for the past two censuses will be the same in taking the next census. Although there may be room for argument about the nature of these assumptions, they are. essentially either the same or quite similar to the assumptions which must be made implicitly in other kinds of projections by age. UNDER THE ASSUMPTIONS STATED ABOVE, THE FORMULA IS EXACT. It does not require explicit separate adjustments for mortality and migration. Better still, the formula has built into it the LOCAL combined mortality and migration rates of the base decade 19501960. Hence, there is no need to adjust for differences between mortality of the local area and that of the state or nation as is necessary in the use of U. S. Census survival rates in estimating net migration in local areas. For the same reason, the basic formula may be used with race and sex classifications without adjustments of any kind. The above formula is more obvious when it is stated in such a manner that, for example, the ratio of white males ages 3034 in 1970 is to that, same group, ages 2024 in i960, as white males ages 3034 in 1960 are to the same group, ages 2024 in 1950. Obviously, the ratios are functions of the effects of migration and mortality operating during the two decades. They may be equal to, more than, or less than unity, depending on the relative amount of net migration and mortality during the decade. in 