Gambar halaman
PDF
ePub

9

[ocr errors]

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.

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.

2. Add to and subtract from 50 percent the standard error determined in step 1.

3. Using the distribution of the characteristic, calculate the confidence interval corresponding to the two points established in step 2.

A two-standard-error confidence interval may be determined by finding the values corresponding to 50 percent plus and minus twice the standard error determined in step 1.

Illustration of the Computation of a
Confidence Interval for a Median

Table 5 of this report indicates that the median travel time to work for commuters who drove alone in 1975-76 was 18.2 minutes.

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

[blocks in formation]
[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]
[merged small][ocr errors][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][ocr errors][subsumed][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small]

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

[blocks in formation]

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

[blocks in formation]
[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

Appendix C-Definitions and Explanations

Most of the terms used in this report are self-explanatory or can best be understood by reference to the appropriate questionnaire items. (See appendix B.) An explanation of other subjects is provided below.

Worker. For purposes of the Travel-to-Work Supplement, a worker is any member of a sample household 14 years old or over who had a regular part-time or full-time job the week prior to interview. A job is defined as a definite arrangement for regular work for pay every week or every month. This included persons who operated their own business, professional practice, or farm. A household member was also considered to be a worker if the person had a regular job, but was temporarily absent from work due to illness, vacation, layoff, etc.

Place of work. This is the actual geographic location at which the worker usually carried out their occupational or job activities. If the person was on a business trip, on vacation, taking classes, etc., the week prior to interview, the person's usual place-of-work location was obtained. Workers who had the type of job in which they worked at one location for a period of time and then changed work locations (e.g., a temporary office worker) were asked to report the location of the first place they worked the previous week. Persons who did not usually work at the same location each day were requested to give the location where they usually reported to begin work each day. Persons who neither worked at the same location nor began work at the same location each day were classified as having no fixed place of work.

No fixed place of work. Workers with no fixed place of work were those who did not usually work at the same location each day and did not usually report to a central location to begin work each day.

Means of transportation to work. Means of transportation refers to the principal mode used to get from home to work. Workers who used different means of transportation on different days of the week were asked to specify the one used most often. Workers who used more than one means of transportation to get to work each day were asked to specify the one used for the longest distance during the work trip.

Automobile. The category "automobile" includes workers using cars, station wagons, company cars, and passenger vans.

Truck. The category "truck" includes workers using pick-up trucks, panel trucks, and other trucks of 1-ton capacity or less. Workers who used larger trucks to get to work are classified as using "other means.”

Travel distance to work. The one-way, "door-to-door" distance in miles that the person reported usually traveling from home to work during the week prior to interview was counted as the travel distance to work. Respondents were instructed to report travel distance rounded to the nearest mile. However, some heaping of the responses did occur; i.e., persons were more likely to report distances of 5, 10, 15, 20, etc., miles than values between these figures. Travel time to work. The total elapsed time in minutes that the person reported it usually took to get from home to work during the week prior to interview was counted as the travel time to work. The elapsed time included time spent waiting for public transportation and picking up members of carpools. Respondents were instructed to report travel time to the nearest minute. However, substantial heaping of the responses did occur; i.e., persons were much more likely to report travel times of 5, 10, 15, 20, 30, and 45 minutes than values between these figures. Some heaping also occurred at 25, 35, and 40 minutes, although not to the same extent. A large proportion of the heaping was presumably due to the daily variation in travel time to work experienced by most workers, plus the manner in which the question was asked ("How long does it usually take to get from home to work?").

Metropolitan areas. The term "metropolitan area" as used in this report refers to the 243 standard metropolitan statistical areas (SMSA's) used in the 1970 census. Changes in SMSA definition criteria, boundaries, and titles made after February 1971 are not reflected in the report.

Except in the New England States, a standard metropolitan statistical area was essentially defined in 1970 as a county or group of contiguous counties containing at least one city of 50,000 inhabitants or more (or "twin cities" with a combined population of at least 50,000). Contiguous counties were included in the SMSA definition if, according to certain criteria, they were socially and economically integrated with the central county. In the New England States, SMSA's consisted of towns and cities instead of counties. Each 1970 census SMSA included at least one central city; the complete title of an SMSA identified the

central city or cities.

7

Central cities. Each 1970 census SMSA included at least one central city. They were determined essentially according to the following criteria:

1. The largest city in an SMSA is always a central city.

2. One or two additional cities may also be named central cities on the basis and in the order of the following criteria:

a. The additional city or cities have at least 250,000 inhabitants.

b. The additional city or cities have a population of one-third or more of that of the largest city and a minimum population of 25,000.

Suburbs or suburban area. That portion of metropolitan areas which is outside of central cities is referred to in the text and tables of this report as "suburbs," "suburban area," or "in SMSA's, outside central cities." The term "suburb" is used here for convenience since for some metropolitan areas the territory outside central cities extends beyond what might reasonably be considered suburban.

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.

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.

Head of household. 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 recognition 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.

[blocks in formation]

113

« SebelumnyaLanjutkan »