Appendix:
Variable Definitions, Sources and Sample Statistics
Population
Growth for the age group, county, and
farm or nonfarm status is measured by the difference in log population over
consecutive census years. Unpublished census
data were used to correct for changes in the definition of a farm that occurred
in 1980.
Median
Income in the county was divided by
average family size. In years when the
income was not reported separately for either the farm population (1950) or the
nonfarm population, the missing data was approximated by solving the equation YT
= "FYF
+ (1 - "F) YNF where YT is median income in the
county, "F is farm
share of total population, and YF (YNF) is farm (nonfarm)
income. This created measurement error
in the estimate of farm and nonfarm income, necessitating the need for
instrumental variable methods to correct for the measurement error.
Human
Capital measures include the percent
of the population aged 25 and over who have completed a high school degree and
the median years of schooling completed for that age group.
Local
Government Policy measures were only available for the second
and seventh years of each decade.
Observations within a decade we averaged so as to generate a
representative estimate of the policy at the midpoint of the decade. Because the policies may change in response
to population changes, these variables are instrumented by exogenous variables
dated at the start of the decade.
Measures of state government tax and
highway expenditure policies are dated at the start of the decade, as are
federal highway funds. These are
treated as exogenous to individual county population growth.
Demographic
Variables are self-explanatory. The proportion of the county that are on
farms, under 15, over 64, or black are taken directly from census data.
Local
amenities and prices include median
gross monthly rent measured at the start of the decade. Separate decade dummy variables control for
changes in prices that are common across rural counties. Measures of county average rainfall and
temperature are included as local amenities.
Herfindahl
employment index is measured by , where Ej is the jth one-digit SIC industry’s
share of total employment in the county.
In log form, the index varies from minus infinity to zero with zero
being the case where all employment is concentrated in one industry.
Distance to the nearest city of over 100,000 was computed from
maps.
Instrumental
variables used were intended to
measure local cost of providing public services such as the suitability of
local soil types for road construction, cost of county rural road construction
per mile, county average teacher pay, and union membership as a percentage of
the labor force in the state. Local
poverty was measured as the percentage of families in the county with income
less than $5000 in 1980 dollars. This
is used as a measure of local taste for public services. To help identify nonfarm versus farm income,
measures of value of farm land and buildings per acre, the proportion of farm
revenues from crop production and average acres per farm.
An
electronic copy of the data may be downloaded from
http://www.econ.iastate.edu/faculty/
orazem/.
Appendix: Sample Statistics
and Data Sourcesa |
|||
ENDOGENOUS VARIABLES |
Source |
Mean |
S.D. |
Population Growth: |
|
|
|
|
|
|
|
Overall 20-64 |
Census of Population |
-.002 |
.18 |
20-34 |
Census of Population |
-.058 |
.27 |
Farm 20-64 |
Census of Population |
-.388 |
.32 |
20-34 |
Census of Population |
-.515 |
.45 |
Nonfarm 20-64 |
Census of Population |
.128 |
.21 |
20-34 |
Census of Population |
.072 |
.27 |
|
|
|
|
Government Policy |
|
|
|
County per capita welfare
expenditure |
Compendium of Government
Finances |
1.30 |
2.68 |
County per capita education
expenditure |
Compendium of Government
Finances |
5.86 |
.52 |
County highway expenditure |
Compendium of Government
Finances |
7.12 |
.98 |
County per capita debt
outstanding |
Compendium of Government
Finances |
5.88 |
1.12 |
County per capita tax
revenue |
Compendium of Government
Finances |
5.47 |
.77 |
|
|
|
|
Median per person income |
|
|
|
Farm |
Census of Population |
8.48 |
.53 |
Nonfarm |
Census of Population |
8.59 |
.41 |
|
|
|
|
EXOGENOUS VARIABLES |
|
|
|
Median income per person |
Census of Population |
8.55 |
.44 |
Median school years
completed |
Census of Population |
2.29 |
.18 |
Percent of population with
high school degree |
Census of Population |
3.56 |
.48 |
Distance to a city with
population > 100,000 |
Computed by authors |
4.47 |
.72 |
Herfindahl index of
employment |
Census of Population |
-1.56 |
.30 |
Rent |
Census of Housing |
5.16 |
.36 |
State government highway
expenditure |
Compendium of Government
Finances |
13.01 |
.58 |
Proportion on farm |
Census of Population |
-1.50 |
.88 |
Proportion black |
Census of Population |
-.63 |
2.57 |
Proportion less than 15
years old |
Census of Population |
-1.28 |
.25 |
Appendix Table (continued) |
Source |
Mean |
S.D. |
Proportion 65 years and
over |
Census of Population |
-2.16 |
.32 |
Average January temperature |
ISU Department of Agronomy |
3.30 |
.57 |
Average July temperature |
ISU Department of Agronomy |
4.34 |
.05 |
Average annual rainfall |
ISU Department of Agronomy |
3.54 |
.40 |
|
|
|
|
Instruments |
|
|
|
Per capita state taxes
(state level) |
State Government
Finances |
5.95 |
.50 |
Percent of low income
households |
Census of Population |
2.92 |
.68 |
Average teacher salary |
Digest of Educational
Statistics |
8.93 |
.31 |
Cost of primary rural road
per mile |
Highway Statistics |
6.31 |
.93 |
Cost of secondary rural
road per mile |
Highway Statistics |
4.93 |
1.04 |
Federal highway funds per
mile (state level) |
Highway Statistics |
3.55 |
.74 |
Percent of union membership |
Statistical Abstract, Union Sourcebook, and Directory of National
Unions and Employee Associations |
3.09 |
.37 |
Soil suitability for local
roads |
Map Unit Interpretation
Record Database |
.27 |
.19 |
Soil suitability for
roadfill |
Map Unit Interpretation
Record Database |
.41 |
.23 |
Percent of farm revenue in
crops |
Census of Agriculture |
3.35 |
.90 |
Average farm size (acres) |
Census of Agriculture |
5.55 |
.86 |
Value of land and building
per acre |
Census of Agriculture |
6.10 |
.80 |
aAll
variables in natural logarithms.