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Consequently, the 68-percent confidence interval for the 2.5 percent difference is from 1.8 to 3.2 percent. Therefore, a conclusion that the average estimate of this difference, derived from all possible samples, lies within a range computed in this way would be correct for roughly 68 percent of all possible samples. Similarly, the 90-percent confidence interval is from 1.4 to 3.6 percent, and the 95-percent confidence interval is from 1.1 to 3.9 percent. Thus, we can conclude with 95-percent confidence that the percentage of female workers who used public transportation in 1975 is greater than the percentage of male workers who used transit, since the 95-percent confidence interval does not include zero or negative values.
Thus, the 95-percent confidence interval on the estimated median is from 17.8 to 18.7 minutes.
ndarder D. Corect
Standard error of an arithmetic mean. The standard error of an arithmetic mean can be approximated by the following formula:
where y is the size of the base, and b is a parameter which equals 140.5 for this SMSA, 146.2 for the central city, and 136.0 for the balance.
Standard error of a median. The sampling variability of an estimated median depends upon the form of the distribution as well as the size of its base. An approximate method for measuring the reliability of a median is to determine an interval about the estimated median, such that there is a stated degree of confidence that the median based on a complete census lies within the interval. The following procedure can be used to estimate the 68-percent confidence limits on sample data: 1. Determine, using the appropriate standard error table, the
standard error of the estimate of 50 percent from the distribution.
The variance, S?, is given by
where c is the number of groups; i indicates a specific group. thus taking on values 1 through c; Pi is the estimated proportion with the characteristic in group i; Zi-7 and Z; are the lower and upper interval boundaries, respectively, for
1-1 + group i; and X i
which is assumed to be the most
2 representative value for the characteristic for persons in group i. Group c is open-ended, i.e., no upper interval boundary exists. For this group an approximate average value is
1. Using table A-2 of the appendix, the standard error of 50
percent on a base of 363,000 is found to be about 1.0
percent. 2. A 95-percent confidence interval on a 50 percent item is
obtained by adding to and subtracting from 50 percent twice the standard error found in step 1. This yields
percent limits 48.0 and 52.0. 3. The median interval is 15 to 24 minutes (14.5 to 24.5). It
can be seen that 34.5 percent of the persons fall in the intervals below the median interval, while 41.5 percent fall in the median interval itself. Thus, the lower limit on the estimate is found to be about
Less than 10 min.
4.5 12.0 19.5 27.0 32.0 42.0 54.5 90.0
14.5+ (24.5 – 14.5)
Table A-1. Standard Errors for Estimated Number of Workers in the San Diego, Calif. SMSA, in the Central City of the
SMSA, and in the Balance of the SMSA
Table A-2. Standard Errors for Estimated Percentage of Workers in the San Diego, Calif. SMSA, in the Central City of the
SMSA, and in the Balance of the SMSA
Other means - Spocity 13. If "Yes" marked in 12a - ASK lif "No" marked in 12a - ASK
Compared to ...'s previous Compared to a you ago, how
I (2) Address (Number and street)
Note - If address (number and street name) are not known, enter building name, shopping center name, or other physical location description.
Much more satisfied
State ZIP code
Most of the terms used in this report are self-explanatory or Truck. The category "truck" includes workers using pick-up can best be understood by reference to the appropriate trucks, panel trucks, and other trucks of 1-ton capacity or questionnaire items. (See appendix B.) An explanation of less. Workers who used larger trucks to get to work are other subjects is provided below.
classified as using "other means." Worker. For purposes of the Travel-to-Work Supplement, a
Travel distance to work. The one-way, "door-to-door" worker is any member of a sample household 14 years old or
distance in miles that the person reported usually traveling over who had a regular part-time or full-time job the week
from home to work during the week prior to interview was prior to interview. A job is defined as a definite arrangement
counted as the travel distance to work. Respondents were for regular work for pay every week or every month. This
instructed to report travel distance rounded to the nearest included persons who operated their own business, pro
mile. However, some heaping of the responses did occur; i.e., fessional practice, or farm. A household member was also
persons were more likely to report distances of 5, 10, 15, 20, considered to be a worker if the person had a regular job, but
etc., miles than values between these figures. was temporarily absent from work due to illness, vacation, Travel time to work. The total elapsed time in minutes that layoff, etc.
the person reported it usually took to get from home to
work during the week prior to interview was counted as the Place of work. This is the actual geographic location at which travel time to work. The elapsed time included time spent the worker usually carried out their occupational or job waiting for public transportation and picking up members of activities. If the person was on a business trip, on vacation, carpools. Respondents were instructed to report travel time taking classes, etc., the week prior to interview, the person's to the nearest minute. However, substantial heaping of the usual place-of-work location was obtained. Workers who had
responses did occur; i.e., persons were much more likely to the type of job in which they worked at one location for a report travel times of 5, 10, 15, 20, 30, and 45 minutes than period of time and then changed work locations (e.g., a values between these figures. Some heaping also occurred at temporary office worker) were asked to report the location 25, 35, and 40 minutes, although not to the same extent. A of the first place they worked the previous week. Persons
large proportion of the heaping was presumably due to the who did not usually work at the same location each day were
daily variation in travel time to work experienced by most requested to give the location where they usually reported to
workers, plus the manner in which the question was asked begin work each day. Persons who neither worked at the
("How long does it usually take to get from home to same location nor began work at the same location each day work?''). were classified as having no fixed place of work.
Metropolitan areas. The term "metropolitan area" as used in
this report refers to the 243 standard metropolitan statistical No fixed place of work. Workers with no fixed place of work
areas (SMSA's) used in the 1970 census. Changes in SMSA were those who did not usually work at the same location
definition criteria, boundaries, and titles made after February each day and did not usually report to a central location to 1971 are not reflected in the report. begin work each day.
Except in the New England States, a standard metro
politan statistical area was essentially defined in 1970 as a Means of transportation to work. Means of transportation
county or group of contiguous counties containing at least refers to the principal mode used to get from home to work.
one city of 50,000 inhabitants or more (or "twin cities" with Workers who used different means of transportation on
a combined population of at least 50,000). Contiguous different days of the week were asked to specify the one
counties were included in the SMSA definition if, according used most often. Workers who used more than one means of
to certain criteria, they were socially and economically transportation to get to work each day were asked to specify
integrated with the central county. In the New England the one used for the longest distance during the work trip.
States, SMSA's consisted of towns and cities instead of
counties. Each 1970 census SMSA included at least one Automobile. The category "automobile" includes workers central city; the complete title of an SMSA identified the using cars, station wagons, company cars, and passenger vans. central city or cities.
Central cities. Each 1970 census SMSA included at least one central city. They were determined essentially according to the following criteria:
A household includes the related family members and all the unrelated persons, such as lodgers, foster children, wards, or employees, who share the housing unit. A person living alone in a housing unit or a group of unrelated persons sharing a housing unit as partners is also counted as a household
1. The largest city in an SMSA is always a central city.
Head of hou sehold. In the 1975-76 Annual Housing Survey, one person in each sample household was designated as the "head." The head of household was defined as the person who was regarded as the head by the members of the household. A married woman was not classified as the head of household if her husband was living with her at the time of the survey.
In the past, the Census Bureau has designated a head of household to serve as the central reference person for the collection and tabulation of data for each member of the household (or family). However, the trend toward recogni. tion of equal status and roles for adult family members makes the term "head" less relevant in the analysis of household and family data. As a result, the Bureau is currently developing new techniques for the enumeration and presentation of data which will eliminate the concept "head." Although the data in this report are based on this concept, methodology for future Census Bureau reports will reflect a gradual movement away from this traditional practice.
Race. Data in this report are provided separately for Black workers, and for White workers and workers of other races combined. Workers in the "White and other races" category are referred to as "White" in the text for convenience. The determination of the race of each worker was based on the observation or inquiry of the enumerator.
Household. A household consists of all the persons who occupy a housing unit. A house, an apartment or other group of rooms, or a single room is regarded as a housing unit when it is occupied or intended for occupancy as separate living quarters; that is, when the occupants do not live and eat with any other persons in the structure and there is either (1) direct access from the outside or through a common hall or (2) a kitchen or cooking equipment for the exclusive use of the occupants.
Earnings. Earnings are the total amount of money earned in the last 12 months by a person working as an employee for a private employer or an incorporated business (including a farm employer or branch of government). Earnings also include such items as piece-rate payments, commissions, tips, cash bonuses, and Armed Forces pay.
Symbols used in this report. A dash "." means "rounds to or represents zero." Three dots "..." means "not applicable."
U.S. GOVERNMENT PRINTING OFFICE: 1979 0_281-042/144